THE     WORLD'S 
MINERALS 


BY 
LEONARD  J.  SPENCER,  M.A.,  F.G.S. 

OF  THE  MINERAL  DEPARTMENT,  BRITISH  MUSEUM 
EDITOR    OF   THE    MINERALOGICAL    MAGAZINE 


WITH  FORTY  COLORED  PLATES  AND  TWENTY-ONE 
DIAGRAMS 


NEW  YORK 
FREDERICK  A.  STOKES  COMPANY 

PUBLISHERS 


5 


H// 


Copyright,  1911,  by 
FREDERICK  A.   STOKES  COMPANY 


EA.      ;t 

SCIENCES 

LIBRARY 


/£//  rights  reserved,  including  that  of  translation  into  foreign 
languages,  including  the  Scandinavian 


GPOtOGICAL  SCIENCES 


PREFACE 

THE  text  of  this  book  on  minerals  is  in  the  main 
descriptive  of  the  116  species  of  the  more  common 
simple  minerals,  which  are  illustrated  by  163  figures 
on  the  colored-plates.  Descriptions  of  a  few  other 
important  species  have  been  added.  Technical  terms 
are  explained  in  the  preliminary  chapters;  their  use, 
however,  has  been  avoided  as  far  as  possible,  and  an 
attempt  has  been  made-to  present  in  popular  language 
an  interesting  and  readable  account  of  the  several 
kinds  of  minerals.  Points  of  general  interest  are 
touched  upon,  and  attention  is  drawn  to  such  of  the 
more  prominent  characters  as  will  fielp  the  student 
and  collector  of  minerals  to  identify  his  own  speci- 
mens. Mention  is  also  made  of  the  various  practical 
applications  of  minerals,  their  importance  as  ores  of 
the  metals,  as  precious  stones,  etc. 

The  forty  colored-plates  have  been  prepared  under 
the  supervision  of  Dr.  Hans  Lenk,  Professor  of  Min- 
eralogy and  Geology  in  the  University  of  Erlangen, 
and  many  of  the  pictures  represent  actual  specimens 
belonging  to  the  mineral  collection  under  his  charge. 

A  correct  idea  of  minerals  cannot,  however,  be 
conveyed  by  pictures  and  descriptions  alone.  It  is 
necessary  to  handle  actual  specimens,  and  if  the 
student  possesses  a  small  collection  of  his  own,  his 
acquaintance  with  the  minerals  will  be  all  the  more 

v 

M160151 


vi  PREFACE 

real.  There  are  few  objects  more  attractive  and 
readily  preserved,  from  a  collector's  point  of  view, 
than  minerals.  Specimens  may  be  acquired  for  a 
small  sum  from  the  mineral  dealers  in  all  large  cities; 
but  if  the  student  can  visit  mining  districts  and  collect 
and  observe  for  himself,  he  will  learn  far  more.  A 
knowledge  of  minerals  is  not  only  of  interest  in  itself, 
but  it  can  often  be  turned  to  useful  account. 

L.  J.  S. 


CONTENTS 

CHAPTER  PAGE 

I.  INTRODUCTION I 

II.  THE  FORMS  OF  MINERALS .6 

Cubic  System    .........  9 

Tetragonal  System 19 

Orthorhombic  System 21 

Monoclinic  System 22 

Anorthic  System 23 

Rhombohedral  System       .         .. 23 

Hexagonal  System 25 

The  Habit  of  Crystals 25 

Intergrowths  of  Crystals 27 

Forms  of  Aggregation  of  Crystals 29 

Amorphous  Minerals 32 

Pseudomorphs 32 

III.  THE  PHYSICAL  CHARACTERS  OF  MINERALS     ....  34 

Colors  of  Minerals -34 

Transparency  and  Luster 37 

Specific  Gravity .38 

The  Fracture  and  Cleavage  of  Minerals  ....       41 

Hardness 43 

Magnetic  and  Electrical  Properties  ...  -44 

Touch,  Taste,  and  Smell   ...  .45 

IV.  THE    CHEMICAL    COMPOSITION    AND    CLASSIFICATION     OF 

MINERALS .         .      46 

V.  THE  NATIVE  ELEMENTS  * 53 

Non-metallic  Elements      .         .  .  -54 

Semi-metallic  Elements     .  •       67 

Metallic  Elements 70 

VI.  THE  SULPHIDES,  ARSENIDES,  AND  SULPHUR-SALTS         .         .       78 

VII.  THE  HALOIDS 101 

VIII.  THE  OXIDES        .  .     no 

IX.  THE  CARBONATES         .         .  .138 

*  For  an  enumeration  of  the  several  species  of  minerals  described  in  Chapters  V.- 
XIV.,  see  the  List  of  Plates  given  on  pages  ix  and  x. 


viii  CONTENTS 


CHAPTER  PACK 

X.  THE   SULPHATES,   CHROMATES,   MOLYBDATES,   AND    TUNG- 
STATES  .        .        .        .        .        .        .        .        .        .156 

XI.  THE  PHOSPHATES,  ARSENATES,  AND  VANDADATES         .        .     170 
XII.  THE  SILICATES     .        .        .        .        .        .        .        .        .183 

The  Felspar  Group   .         .        .         .        ,         .         ,         .186 

The  Amphibole  Group     ,        ,        .        .  •      .         .         .     195 

The  Pyroxene  Group 202 

The  Sodalite  Group        ...        .        .        .        .         .207 

The  Garnet  Group    .         ...        .        .        .         .     213 

The  Mica  Group 233 

The  Chlorite  Group .239 

The  Zeolite  Group    .        .        .•••       .         .         .     244 

XIII.  THE    TlTANO-SILICATES,    TlTANATES,   AND    NlOBATES        .  .      249 

XIV.  THE  ORGANIC  SUBSTANCES  .        .        .        .        .        ,        .252 
INDEX  261 


LIST  OF  PLATES 

FACING 
PAGE 

Plate    i.  NON-METALLIC  ELEMENTS:  Diamond,  Graphite,  Sulphur  62 
„       2.  METALLIC    ELEMENTS  :     Arsenic,    Antimony,      Bismuth, 

Silver 70 

„       3.            „                 „               Platinum,  Gold,  Copper    .         .  74 

„       4.  SULPHIDES  :  Stibnite,  Realgar,  Orpiment  ....  80 

„       5.            „            Molybdenite,  Zinc-Blende,  Galena         .         .  84 
„       6.  SULPHIDES   AND   ARSENIDES:    Niccolite,   Cinnabar,   Mis- 

pickel     ....  88 

„       7.  SULPHIDES  :  Marcasite,  Iron-pyrites,  Pyrrhotite        .         .  94 
„       8.  SULPHUR-SALTS,    &c. :    Copper-pyrites,    Smaltite,    Tetra- 

hedrite,  Pyrargyrite,  Proustite  98 

„       9.  HALOIDS:  Fluor-spar 102 

„     10.  HALOIDS  AND  OXIDES  :  Rock-salt,  Atacamite,  Opal  .         .  108 
„      ii.  OXIDES     (Quarts     Group}  :     Amethyst,     Smoky-quartz, 

Rock-crystal,  Cat's-eye,  Rose-quartz       .         .114 

„      12.        „          (Quartz  Group)  :  Agate,  Jasper,  Hornstone      .  116 

„      13.        „         Haematite 122 

„     14.        „         Magnetite,  Corundum,  Cassiterite,  Zircon,  Pitch- 
blende, Limonite    ......  130 

„     15.        „         Manganite,  Pyrolusite,  Psilomelane    .         .         .  134 

„     16.  CARBONATES:  Calcite 138 

„      17.  „  Calamine,         Chalybite,        Rhodochrosite, 

Cerusite 144 

„      18.            „               Aragonite,  Cerusite 150 

„      19.            „               Chessylite,    Malachite        ....  154 

„     20.  SULPHATES:  Barytes,  Anglesite,  Celestite        .         .         .  160 

„     21.           „             Gypsum,  Linarite 164 

„     22.  CHROMATES,    TUNGSTATES,  &c. :    Crocoite,    Wolframite, 

Wulfenite,  Scheelite                                 .  168 
„     23.  PHOSPHATES,   &c. :    Apatite,    Pyromorphite,    Vanadinite, 

Erythrite 174 

„     24.  PHOSPHATES:  Wavellite,    Lazulite,    Cuprouranite,    Vivi- 

anite,  Turquoise 178 

„     25.  SILICATES    (Felspar   Group)  :    Orthoclase,     Labradorite, 

Microcline,  Anorthite         ....  190 
ix 


LIST    OF    PLATES 


PAGE 

Plate  26.  SILICATES  :  (Amphibole  Group)  :  Hornblende,  Actinolite, 

Crocidolite,  Nephrite  ....     198 

„     27.          „          (Pyroxene  Group)  :  Augite,  Diopside,  Bronzite, 

„     28.          „          Sodalite,  Lapis-lazuli,  Leucite,  Beryl      .        .     208 

Hypersthene,  Wollastonite          .         .        .     204 

„     29.          „          Garnet,  Olivine,  Idocrase        .         .         .        .     216 

„     30.          „          Topaz  220 

„     31.          „          Andalusite  (Chiastolite),  Kyanite    .         .         .224 
„     32.          „          Epidote,  Axinite,  Prehnite    .  ...     226 

„     33.          „          Tourmaline,  Staurolite 232 

„     34.          „  (Mica  and  Chlorite  Groups')  :  Muscovite,  Biotite, 

Zinnwaldite,  Lepidolite,  Clinochlore  .  .  238 
„  35.  „  Serpentine,  Meerschaum,  Talc  .  .  .  240 
„  36.  „  (Zeolite  Group) ;  Analcite,  Chabazite  .  .  244 
„  37-  „  „  Natrolite,  Stilbite  .  .  248 

„     38.  TITA NO-SILICATES,    NIOBATES,    &c.  I    Sphene,    Columbite, 

Perovskite  .         .     250 
„     39.  ORGANIC  SUBSTANCES:  Amber  (enclosing  insect),  Asphal- 

tum,  Lignite     ....     254 
„     40.         „  „  Coal,  Anthracite    .        .        .        .258 


FIGURES  IN  THE  TEXT 

PAGE 

Fig.  i.  The  Octahedron,  with  Axes  of  Reference  10 

„     2.  The  Cube,  with  Axes  of  Reference 10 

,,     3.  Net  for  the  construction  of  a  Model  of  the  Octahedron     .  n 
„    4.  Net  for  the  construction  of  a  Model  of  the  Cube        .         .  n 
„     5.  The  Rhombic-dodecahedron,  with  Axes  of  Reference        .  13 
,.    6.  Net  for  the  construction  of  a   Model   of  the   Rhombic- 
dodecahedron            -13 

,,     7-9.  Crystals  consisting  of  the  Cube  and  the    Octahedron    in 

Combination 15 

,,     10,  ii.  Mis-shapen  Octahedra 17 

,,     12,  13.  Mis-shapen  Cubes        .......  17 

„     14,  15.  Tetragonal  Bipyramids 19 

„     16.  Tetragonal  Prism  with  Basal  Planes          ....  19 

„     17.  Rhombic  Bipyramid 21 

„     18.  Rhombic   Prism  with  Pinacoid 21 

„     19.  Acute   Rhombohedron 24 

„     20.  Obtuse  Rhombohedron     ...          ....  24 

„     21.  Hexagonal  Prism  with  Basal  Planes         ....  24 


xi 


THE    WORLD'S    MINERALS 


THE  WORLD'S  MINERALS 


CHAPTER  I 

INTRODUCTION 

THE  natural  objects  found  on  the  surface  of  our 
earth,  or  within  its  crust  at  a  moderate  distance  from 
the  surface,  may  be  conveniently  classified  into  three 
great  groups  or  kingdoms — the  animal,  vegetable, 
and  mineral  kingdoms.  The  two  first  embrace  every- 
thing organic — that  is  to  say,  everything  that  is,  or 
has  been,  endowed  with  life;  these  are  largely  made 
up  of  the  chemical  element  carbon  combined  with 
the  elements  (hydrogen  and  oxygen)  of  water.  The 
mineral  kingdom,  on  the  other  hand,  includes  all  in- 
organic bodies  that  have  not  been  produced  by  the 
agency  of  life.  Contrasted  with  materials  of  organic 
origin,  they  show  a  much  greater  diversity  in  the 
chemical  elements  that  enter  into  their  composition. 
A  little  consideration  will  show  that  a  compre- 
hensive study  of  the  mineral  kingdom  is  not  so  sim- 
ple a  matter  as  might  at  first  sight  appear.  About 
a  century  ago  this  study  was  known  as  mineralogy, 
but  at  the  present  day  it  is  divided  up  into  a  number 
of  distinct,  though  still  related,  sciences.  The  geolo- 
gist studies  the  wider  questions  relating  to  the  forma- 
tion and  subsequent  destruction  of  the  rocks  of  the 

1 


/     .    THE   WORLD'S    MINERALS 


earth's  crust;  the  petrologist  determines  the  different 
kinds  of  rocks  and  their  relations  to  one  another;  the 
palaeontologist  studies  the  mineralized  remains  of  ex- 
tinct animals  and  plants  found  embedded  in  rocks; 
finally,  the  mineralogist  examines  the  various  kinds 
of  simple  or  individual  materials  which  enter  into 
the  composition  of  the  earth's  crust,  or,  as  we  may 
say,  the  A  B  C  of  the  mineral  kingdom.  Some  other 
branches  of  inquiry,  such  as  those  relating  to  ore- 
deposits,  building,  ornamental,  and  precious  stones, 
are  also  now  coming  to  be  studied  as  more  or  less 
special  subjects. 

It  is,  of  course,  not  always  possible  to  draw  a  hard- 
and-fast  line  between  the  animal,  vegetable,  and 
mineral  kingdoms.  To  take  a  simple  example :  am- 
ber is  usually  treated  of  as  a  mineral,  and  as  such 
finds  a  place  in  the  present  volume;  but  it  is  regarded 
by  the  botanist  as  a  fossil  resin,  and  it  may  be  of  in- 
terest to  the  zoologist  also  on  account  of  the  insects 
which  it  encloses.  Again,  pearls  and  coral,  though 
consisting  of  material  chemically  identical  with  the 
minerals  aragonite  and  calcite,  and  further  ranking 
with  the  precious  stones,  are  clearly  products  of  the 
animal  kingdom.  In  such  cases  there  is  a  certain 
amount  of  overlapping;  but  it  is  no  disadvantage  to 
have  the  same  objects  studied  from  the  different 
points  of  view  of  the  zoologist,  botanist,  and  miner- 
alogist. These,  indeed,  are  not  the  only  points  of 
view  from  which  such  objects  may  be  regarded.  The 
man  of  commerce  looks  upon  amber,  for  instance,  as 
valued  at  so  much  a  pound,  and  he  watches  with 


INTRODUCTION  3 

interest  the  fluctuations  in  the  supply  and  demand  of 
the  raw  material.  The  practical  worker  of  amber 
studies  each  piece  with  a  view  to  making  the  most 
of  his  available  material  in  fashioning  some  object 
of  utility.  Still  again,  we  may  have  the  legal  aspect; 
and  only  recently  the  law  courts  have  been  called 
upon  to  decide  what  is  amber  and  what  is  not  amber, 
and  whether  china-clay  is  or  is  not  a  mineral. 

It  would,  indeed,  be  difficult  to  frame  a  definition 
of  a  mineral  which  would  be  satisfactory  from  every 
point  of  view.  But  as  students  of  nature,  it  is  desir- 
able that  we  should  endeavor  to  obtain  a  clear  con- 
ception of  what  we  may  call  a  simple  mineral,  or  a 
mineral  species. 

As  examples  of  mineral  substances  met  with  in 
everyday  life,  we  may  mention  the  following  kinds 
of  stones:  granite  as  used  for  curb-stones,  basalt  used 
for  macadam  roads,  marbles  used  for  statuary  and 
the  decoration  of  buildings ;  many  others  will  at  once 
present  themselves  to  the  reader. 

If  we  examine  closely  a  piece  of  granite,  we  at 
once  see  that  it  is  made  up  of  more  than  one  kind  of 
material;  and  if  we  crush  it  to  a  coarse  powder,  we 
can  pick  out  fragments  of  three  kinds,  which  to  the 
unaided  eye  present  quite  different  appearances. 
These  three  kinds  of  material  are  called  quartz,  fel- 
spar, and  mica.  If  we  take  them  separately  and  crush 
them  to  a  still  finer  powder,  each  kind  will  still 
present  exactly  the  same  characters  as  before;  and  it 
is  not  possible  by  mechanical  means  to  resolve  them 
into  material  presenting  other  characters.  It  is  true 


4  THE   WORLD'S   MINERALS 

that  the  chemist  by  analytical  methods  can  still  fur- 
ther resolve  them  into  two  or  more  (and  in  the  case 
of  mica  as  many  as  a  dozen)  chemical  elements;  but 
by  so  doing  he  destroys  the  original  nature  of  the 
material  and  all  the  characters  that  appertain  to  it. 

Taking  now  our  next  example,  that  of  basalt:  To 
the  unaided  eye  this  appears  to  consist  throughout  of 
only  one  kind  of  material,  or,  in  other  words,  it  is 
apparently  homogeneous.  When,  however,  a  slice  is 
cut  so  thin  that  it  is  transparent,  and  this  is  examined 
under  the  microscope,  it  is  at  once  apparent  that  here 
also  we  have  a  mixture  of  materials.  In  the  case  of 
marble,  on  the  other  hand,  we  find  that  this  consists 
entirely  of  a  single  kind  of  material,  which  is  the  min- 
eral calcite;  and  in  a  white  statuary  marble  the  in- 
dividual crystalline  grains  of  calcite  are  visible  to  the 
unaided  eye. 

Granite  and  basalt,  therefore,  though  mineral  sub- 
stances, consist  of  mixtures  of  different  kinds  of  min- 
erals. Such  mineral  mixtures  are  known  as  rocks, 
and  the  ultimate  kinds  of  minerals  of  which  they  are 
composed  are  known  as  simple  minerals  or  mineral 
species.  The  term  rock  may  also,  for  convenience,  be 
applied  to  marble,  since  it  occurs  in  nature  in  very 
large  masses  under  the  same  conditions  as  other  rock- 
masses;  if  it  were  found  only  in  small  masses  it  would 
be  merely  regarded  as  a  compact  variety  of  the  min- 
eral species  calcite. 

In  the  following  descriptions  of  the  different  kinds 
of  minerals — that  is,  of  the  various  mineral  species — 
we  shall  find  that  each  species  possesses  certain  es- 


INTRODUCTION 


sential  characters  that  are  always  the  same,  and  that 
these  are  characteristic  of  and  peculiar  to  each  kind 
of  matter  or  stuff  (Stoff ',  as  so  expressively  used  in 
German). 

Before,  however,  passing  to  the  more  detailed  de- 
scription of  the  species  of  minerals  represented  on  the 
colored  plates,  it  will  be  necessary  to  give,  at  least 
briefly,  some  account  of  the  characters  of  minerals 
in  general  (Chapters  II-IV).  Here,  also,  will  be 
given  some  explanation  of  the  few  technical  terms 
which  are  employed  in  the  descriptive  portion. 


CHAPTER  II 

THE  FORMS  OF  MINERALS 

WHEN  minerals  grow  freely  in  rock-cavities  they  as- 
sume of  their  own  accord  certain  external  shapes. 
These  forms  have  the  appearance  of  artificially-made 
geometrical  solids,  being  bounded  by  plane  surfaces 
which  intersect  in  straight  lines,  and  they  are  known 
as  crystals.  Crystals  are,  however,  not  confined  to  the 
mineral  kingdom,  for  many  other  substances,  both 
inorganic  and  organic,  have  the  power  of  shaping 
themselves  into  such  regular  forms. 

It  is  quite  an  easy  experiment  to  cause  the  growth 
of  crystals  of  alum,  common  salt,  saltpetre,  sugar,  etc., 
by  simply  dissolving  these  substances  in  water  and 
allowing  the  solution  to  evaporate.  The  best  method 
of  proceeding  is  to  add  to  boiling  water  as  much  of 
the  substance  as  will  dissolve,  and  then  to  pour  the 
concentrated  solution  into  a  saucer;  as  the  liquid  cools 
some  of  the  material  held  in  solution  will  separate  out 
in  the  form  of  bright,  sparkling  crystals  on  the  bot- 
tom of  the  saucer.  Larger  and  better-shaped  crystals 
can  be  obtained  by  taking  one  of  those  grown  in  the 
saucer  and  suspending  it  by  a  thread  in  a  concentrated 
solution  of  the  same  substance;  the  liquid  is  then  put 
aside  in  a  room  where  the  temperature  is  fairly  con- 
stant, and  allowed  to  evaporate  slowly  for  some  days. 

6 


THE    FORMS    OF    MINERALS  7 

Experimenting  in  this  way  with  different  sub- 
stances that  are  soluble  in  water  or  other  liquids,  we 
may  get  together  an  interesting  and  instructive  col- 
lection of  crystals.  It  will  then  be  recognized  that 
the  crystals  of  different  substances  are  different  in 
shape,  each  substance,  in  fact,  crystallizing  in  its  own 
fashion  or  form.  Alum,  for  instance,  gives  rise  to 
octahedra,  common  salt  to  cubes,  and  sugar  to  tabular 
forms  of  rectangular  outline  (as  in  the  well-known 
sugar-candy) .  Amongst  minerals,  also,  each  kind  as- 
sumes a  form  of  its  own;  and  it  is  thus  possible  to 
distinguish  one  kind  of  mineral  from  another  from 
a  consideration  of  the  shapes  of  their  crystals. 

The  problem  is,  however,  not  always  an  easy  one, 
for  there  being  an  almost  endless  number  of  minerals 
and  other  chemical  compounds  known  to  mineral- 
ogists and  to  chemists,  so  there  must  be  an  almost  end- 
less variety  in  crystals.  The  problem  is  further  com- 
plicated by  the  fact  that  the  crystals  of  one  and  the 
same  substance  are  not  always  exactly  the  same  in 
their  forms,  although  these  can  be  reduced  to  the 
same  type  or  fundamental  form;  the  common  mineral 
calcite,  for  example,  presents  hundreds  of  different 
forms  of  crystals. 

It  is  the  business  of  the  crystallographer  to  study 
the  different  kinds  of  crystals  and  endeavor  to  classify 
the  large  mass  of  known  facts  respecting  them.  The 
serious  study  of  crystallography  (the  science  of  crys- 
tals) dates  from  the  end  of  the  eighteenth  century, 
and  during  the  early  part  of  the  nineteenth  century 
the  fundamental  laws  of  the  science  were  slowly  ar- 


8  THE    WORLD'S    MINERALS 

rived  at  by  patient  study.  It  has  been  found  that  all 
crystals  can  be  grouped  into  thirty-two  classes,  ac- 
cording to  the  different  degree  of  symmetry  they 
possess ;  and  that  these  classes  fall  into  seven  systems. 
It  is  not  necessary  in  this  place  to  give  an  account 
of  the  thirty-two  classes  of  crystals,  but  the  seven 
systems  are  of  prime  importance.  The  latter  may  be 
defined  by  the  relative  lengths  and  angles  of  inclina- 
tion of  a  set  of  axes  of  reference,  or  crystallographic 
axes,  which  we  are  to  imagine  drawn  inside  the  crys- 
tals. It  will  be  convenient  (for  purposes  of  reference 
alone)  to  give  a  summary  of  the  seven  systems  of 
crystals  in  this  place,  and  later  to  explain  the  matter 
more  fully. 

1.  CUBIC  SYSTEM,  with  three  axes,  all    at    right 
angles  to  one  another,  and  of  equal  lengths. 

2.  TETRAGONAL  SYSTEM,  with  three  axes,  all  at 
right  angles  to  one  another,  two  being  of  equal  lengths 
arid  the  third  either  longer  or  shorter. 

3.  ORTHORHOMBIC  SYSTEM,  with  three  axes,  all  at 
right  angles  to  one  another,  and  all  of  unequal 
lengths. 

4.  MONOCLINIC  SYSTEM,  with  two  axes  inclined  to 
one  another,  but  both  at  right  angles  to  a  third  axis, 
the  three  axes  being  of  unequal  lengths. 

5.  ANORTHIC  SYSTEM,  with  three  axes,  all  inclined 
at  oblique  angles,  and  all  of  unequal  lengths. 

6.  RHOMBOHEDRAL  SYSTEM,  with  three  equal  axes, 
mutually  inclined  at  the  same  angles,  which  are  not 
right  angles. 

7.  HEXAGONAL  SYSTEM,  with  three  equal  axes,  in- 


CRYSTALS— THE  CUBIC  SYSTEM  9 

clined  to  one  another  at  60°  in  one  plane,  and  a  fourth 
axis  of  different  length  perpendicular  to  this  plane. 

CUBIC  SYSTEM 

It  will  be  necessary,  to  begin  with,  to  explain 
briefly  some  of  the  fundamental  principles  of  crystal- 
lography, and  for  this  reason  our  description  of  the 
cubic  system  will  be  longer  than  that  of  any  of  the 
other  six  systems.  Consider  a  regular  octahedron 
(Fig.  i) — that  is,  a  solid  bounded  by  eight  equilat- 
eral triangles;  by  joining  the  opposite  corners  we 
obtain  within  the  solid  three  lines,  AA',  BB',  and 
CC',  of  equal  lengths,  which  intersect  at  right  angles 
in  the  center  O,  of  the  crystal.  These  three  equal 
rectangular  axes  are  the  axes  of  reference,  or  crystal- 
lographic  axes,  of  the  cubic  system. 

Again,  taking  a  cube  (Fig.  2) — that  is,  a  solid 
bounded  by  six  square  planes  at  right  angles  to  one 
another  in  three  pairs;  by  joining  the  centers  of  op- 
posite faces  we  obtain  the  same  set  of  three  equal 
rectangular  axes,  which  are  here  parallel  to  the  edges 
of  the  cube. 

The  cube  and  the  octahedron  are  thus  referable  to 
the  same  set  of  axes.  (In  Figs,  i  and  2  the  axes  are 
drawn  of  the  same  lengths  for  both  solids.)  But 
this  is  not  the  only  point  they  have  in  common ;  they 
also  possess  the  same  degree  of  symmetry.  From  Fig. 
i  it  is  easy  to  see  that  the  octahedron  can  be  cut  along 
each  of  the  three  planes  ABA'B',  ACA'C'  BCB'C' 
into  two  equal  and  similar  halves.  If  one  of  these 


10 


THE    WORLD'S    MINERALS 


halves  be  placed  with  its  cut  surface  against  a  mirror, 
the  mirror-reflection  will  reproduce  the  missing  half, 
and  the  solid  will  appear  complete.  These  planes, 
ABA'B',  etc.,  are  known  as  planes  of  symmetry  of  the 
solid.  Now  a  comparison  of  Figs,  i  and  2  will  show 
that  these  three  planes  of  symmetry  are  respectively 
parallel  to  the  three  pairs  of  parallel  faces  of  the 
cube ;  and  planes  passing  through  the  points  bearing 


Fig.  i— The  Octahedron, 
with  Axes  of  Reference. 


Fig.  2— The  Cube, 
with  Axes  of  Reference. 


the  same  letters  in  Fig.  2  will  also  cut  the  cube  into 
two  equal  halves.  In  addition  to  these  three  prin- 
cipal planes  of  symmetry  of  the  cube  and  the  octa- 
hedron, there  are  in  each  solid  six  other  planes  of 
symmetry;  these  can  be  made  out  by  a  little  study 
from  the  figures,  or,  better  still,  with  the  aid  of 
models. 

Models  are,  indeed,  indispensable  aids  to  the  study 
of  crystals,  and  some  of  the  more  simple  forms  should 
be  made  by  the  reader  himself  if  he  wishes  to  gain 
much  insight  into  this  most  interesting  subject.  Fig. 


SYMMETRY    OF    CRYSTALS 


11 


3  gives  the  surface  of  an  octahedron  unfolded,  or  de- 
veloped, in  a  plane.  A  tracing  of  this  may  be  pasted 
on  thin  cardboard,  cut  out,  and  folded  along  the 
dotted  lines;  the  result  will  be  an  octahedron.  In 
the  same  way  Fig.  4  will  give  a  cube.  When  the 
sheets  are  folded,  the  edges  of  the  models  may  be 
fixed  with  gummed  paper. 

With  the  aid  of  the  models,  it  will  readily  be  seen 
that  these  solids  possess  not  only  nine  planes  of  sym- 
metry, but  also  a  number  of  axes  of  symmetry. 


Fig.  3 — Net  for  the  construction 
of  a  Model  of  the  Octahedron. 


Fig.  4 — Net  for  the  construction 
of  a  Model  of  the  Cube. 


Standing  the  octahedron  up  on  one  corner,  C',  and 
rotating  it  about  the  vertical  axis,  CC',  through  a 
quarter  of  a  complete  revolution — that  is,  so  that  the 
point  A  comes  into  the  position  of  the  point  formerly 
occupied  by  B — we  find  that  the  relative  position  and 
aspect  of  the  solid  is  exactly  the  same  as  before;  and, 
further,  this  covering  position  is  obtained  four  times 
during  a  complete  revolution.  Such  an  axis  of  sym- 
metry is  called  a  tetrad  axis  of  symmetry;  and  it  will 
be  readily  seen  that,  both  in  the  octahedron  and  the 
cube,  there  are  three  such  axes.  Both  solids  also 


12          THE   WORLD'S    MINERALS 

possess  four  axes  of  triad  symmetry  (rotation  about 
which  gives  three  coincident  positions  during  a  com- 
plete revolution),  and  six  dyad  axes  (giving  only 
two  coincident  positions).  In  addition  to  these  nine 
planes  of  symmetry  and  thirteen  axes  of  symmetry, 
there  is  also  a  center  of  symmetry  (O  in  Figs,  i  and 
2),  through  which  every  point  on  the  solid  has  a 
corresponding  point  reflected  at  an  equal  distance  on 
the  other  side.  With  the  help  of  his  home-made 
models  the  student  should  be  able  to  make  out  to  his 
own  satisfaction  all  these  symmetrical  relations  of 
the  cube  and  the  octahedron. 

We  thus  see  that  the  cube  and  the  octahedron, 
though  so  very  different  in  form,  are  both  referable 
to  the  same  system  of  crystals — the  cubic  system.  But 
these  are  not  the  only  forms  of  this  system ;  there  are 
many  others. 

Studying  again  Figs,  i  and  2  from  a  somewhat 
different  point  of  view,  we  see  that  each  face  of  the 
octahedron  intersects  all  the  three  axes  of  reference 
at  equal  distances  from  the  center.  The  face  ABC, 
for  example,  cuts  the  axes  at  the  equal  distances  OA, 
OB,  and  OC.  The  position  of  this  face,  with  refer- 
ence to  these  axes,  is  therefore  expressed  by  the  sym- 
bol (in)  (one,  one,  one).  On  the  other  hand,  the 
faces  of  the  cube  each  intersect  only  one  axis  (at  the 
distance  OA,  OB,  or  OC),  and  are  each  parallel  to 
the  other  two  axes — that  is,  intersect  it  at  an  infinite 
distance.  The  position  of  a  face  of  the  cube  with 
reference  to  the  axes  is  thus  expressed  by  the  symbol 
(i  oooo)  (one,  infinity,  infinity),  or,  if  we  take  the 


THE   FACES   OF   CRYSTALS 


13 


inverse  of  these,  by  the  symbol  (100)   (one,  naught, 
naught). 

Now  let  us  see  what  happens  if  we  take  a  face  with 
the  symbol  (no) — that  is,  a  face  that  intersects  two 
axes  at  equal  (unit)  lengths  and  is  parallel  to  the 
third.  We  shall  find  that  twelve  faces  of  this  kind 
can  be  built  up  around  the  cubic  axes,  and  the  solid 


Fig.  5— The  Rhombic  Dode- 
cahedron, with  Axes  of 
Reference. 


Fig.  6 — Net  for  the  construction  of 
a  Model  of  the  Rhombic-Dodecahe- 
dron.* 


we  arrive  at  is  known  as  the  rhombic-dodecahedron 
(Fig.  5),  which  is  bounded  by  twelve  equal  rhomb- 
shaped  faces.  Continuing  our  exercise  in  paper  cut- 

*A  larger  model,  more  convenient  for  study,  may  be  made  by  draw- 
ing this  net  on  a  larger  scale ;  but  it  is  important  that  the  angles  of  the 
rhombs  should  be  exactly  70^°  and  109^°,  and  that  the  edges  be  all 
equal. 


14          THE    WORLD'S    MINERALS 

ting  and  folding,  we  shall  find  that  a  model  (Fig.  6) 
of  this  solid  possesses  all  the  symmetrical  relations 
of  the  cube  and  the  octahedron. 

Another  variation  in  the  intersection  of  faces  on 
the  axes  may  be  introduced  by  taking  fractional 
lengths  on  one  or  two  of  the  axes.  Thus  a  face  may 
intersect  two  of  the  axes  at  the  unit  length  (OA), 
and  the  third  axis  at  one  half  (or  one  third,  etc.)  this 
distance  from  the  center.  We  shall  then  derive  a 
more  complex  form  with  twenty-four  faces,  for 
which  the  symbol  is  (112)  (or  113,  etc.),  which  is 
known  as  an  icositetrahedron.  Proceeding  in  this 
way,  we  may  obtain  in  the  cubic  system  seven  differ- 
ent kinds  of  simple  forms,  the  most  complex  being 
bounded  by  forty-eight  faces,  all  of  which  are  sym- 
metrically arranged  as  before. 

Now  in  actual  crystals  these  simple  forms  may  ex- 
ist either  alone  or  in  combination  with  each  other. 
It  may  thus  happen  that  one  and  the  same  crystal 
may  be  bounded  by  hundreds  of  small  faces;  but  it 
will  be  found  that  they  all  obey  the  definite  laws  of 
whole  or  fractional  intercepts  on  the  axes,  and  that 
all  are  arranged  with  regularity  as  required  by  the 
planes  and  axes  of  symmetry  noted  above. 

To  give  a  simple  illustration  of  this  combination 
of  forms,  we  may  suppose  that  the  corners  of  the  octa- 
hedron are  truncated,  or  chopped  off,  by  faces  paral- 
lel to  the  planes  AA'BB',  AA'CC',  and  BB'CC'— 
that  is,  parallel  to  the  faces  of  the  cube;  the  result 
will  then  be  as  shown  in  Fig.  7.  Or,  again,  we  may 
truncate  the  eight  corners  of  the  cube  by  triangular 


MALFORMATION    OF    CRYSTALS     15 

faces  parallel  to  the  eight  faces  of  the  octahedron 
(Fig.  9).  By  increasing  the  size  of  the  small  square 
faces  in  Fig.  7,  or  by  increasing  the  triangular  faces 
in  Fig.  9,  we  arrive  at  the  form  shown  in  Fig.  8, 
which  is  known  as  a  cubo-octahedron.  These  three 
crystals  are  therefore  essentially  the  same,  differing 
only  in  the  relative  sizes  of  the  faces  of  the  cube  and 
octahedron.  The  ingenious  reader  will  find  instruc- 
tion in  making  models  of  these  crystals. 

So  far,  we  have  considered  crystals  to  be  ideally 
perfect  in  their   development,   but   actual   crystals 


Fig.  7  F'g-  8  Fig.  9 

Crystals  consisting  of  the  Cube  and  the  Octahedron  in  combination. 

as  found  in  nature  very  frequently  display  some 
irregularity  or  distortion.  This  irregularity  is,  how- 
ever, more  apparent  than  real,  and  depends  only  on 
the  relative  sizes  of  the  faces  and  the  distances  of 
these  from  the  center  of  the  crystal.  However  much 
a  face  may  be  displaced,  it  always  remains  parallel 
to  its  true  position;  consequently  the  angle  between 
two  adjacent  faces  is  always  the  same.  This  law  of 
the  constancy  of  angles  is  one  of  the  fundamental 
laws  of  crystallography. 

Any  apparent  irregularities  are  due  merely  to  ac- 
cidents of  growth.    The  crystal  may  have  grown  in 


16          THE   WORLD'S    MINERALS 

a  cramped  position,  and  material  for  its  growth  may 
not  have  been  supplied  to  it  on  all  sides  at  the  same 
rate.  Thus  the  crystals  of  alum  which  we  caused 
to  grow  in  a  saucer  rested  on  the  bottom,  and  there- 
fore could  not  grow  downwards,  but  only  upwards 
and  sideways;  as  a  result  of  this,  many  of  these  crys- 
tals will  be  of  a  flattened  shape  (Fig.  10).  When, 
however,  we  grow  an  alum  crystal  freely  suspended 
in  its  mother-liquid,  material  for  its  growth  is  sup- 
plied equally  on  all  sides,  and  a  crystal  of  geometric- 
al regularity  is  formed. 

As  illustrations  of  this  malformation  of  crystals, 
we  may  give  the  somewhat  extreme  cases  shown  in 
the  accompanying  figures  (Figs.  10-13).  In  Fig.  10 
we  have  an  octahedron  flattened  parallel  to  one  pair 
of  opposite  and  parallel  faces ;  here  the  material  for 
the  growth  of  the  crystal  was  supplied  mainly  at  the 
edges,  or  the  crystal  may  have  started  its  growth  in  a 
narrow  fissure  in  solid  rock.  In  Fig.  1 1  the  octahe- 
dron has  grown  more  in  the  direction  of  an  edge 
between  two  adjacent  faces.  Similarly,  Figs.  12  and 
13  represent  distorted  forms  of  the  cube.  A  good 
example  of  distortion  is  shown  by  the  crystals  of 
sodalite  in  Plate  28.  Fig.  i;  here  six  faces  of  the 
rhombic-dodecahedron  are  elongated  in  the  direction 
of  a  triad  axis  to  give  a  form  resembling  a  hexagonal 
prism. 

Although  not  geometrically  perfect,  such  crystals 
as  these  are  crystallographically  perfect.  A  com- 
parison of  Figs.  10-13  with  the  perfectly  shaped  octa- 
hedron and  cube  in  Figs,  i  and  2  will  show  that  all 


MALFORMATION    OF    CRYSTALS     17 


the  edges  between  corresponding  faces  of  these  crys- 
tals retain  their  true  directions,  while  the  angles  be- 
tween corresponding  edges  and  faces  are  the  same. 


Fig.  10  Fig.  ii 

Misshapen  Octahedra. 

It  might  be  said  that  these  distorted  forms  are  not 
symmetrical  with  respect  to  the  planes  and  axes  of 
symmetry  mentioned  above ;  but  the  symmetrical  re- 
lations of  the  angles  are  preserved,  and  so  is  the 
regularity  in  the  internal  structure  of  the  crystal. 


Fig.  12 


Misshapen  Cubes. 


Fig.  13 


An  important  result  of  this  parallelism    of    the 
edges  of  crystals  is  that  their  faces  are  arranged  in 
In  other  words,  there  is  a  girdle  of  faces,  the 


zones. 


18          THE   WORLD'S   MINERALS 

edges  of  intersection  of  which  are  parallel.  This  is 
best  shown  by  the  nets  of  the  cube  (Fig.  4)  and  the 
rhombic-dodecahedron  (Fig.  6)  ;  it  will  be  seen  that 
in  the  cube  there  is  a  continuous  strip  of  four  faces, 
and  in  the  rhombic-dodecahedron  one  of  six  faces. 
Again,  in  Fig.  7,  we  see  that  four  octahedron  faces 
and  two  cube  faces  lie  in  a  zone,  since  their  edges  of 
intersection  are  parallel;  and  on  this  crystal  there  are 
six  zones  of  this  kind. 

Turning  now  to  the  figures  on  the  colored  plates, 
we  shall  find  many  examples  from  actual  mineral 
specimens  of  the  forms  of  crystals  belonging  to  the 
cubic  system,  of  which  a  brief  sketch  has  been  given 
above. 

Minerals  that  crystallize  in  the  form  of  the  octa- 
hedron are  illustrated  by  diamond  (Plate  i,  Fig.  i) 
and  magnetite  (14,  i)  ;  those  in  the  cube  by  native 
copper  (3,5),  iron-pyrites  (7,2),  fluor-spar  (9,  i  and 
2),  rock-salt  (10,  i  and  2),  and  perovskite  (38,  4). 
Combinations  of  the  cube  and  the  octahedron  are 
shown  by  galena  (5,  3  and  4)  and  smaltite  (8,  3). 
The  icositetrahedron  with  the  symbol  (112)  is  shown 
by  crystals  of  analcite  (36,  i)  and  leucite  (28,  3), 
and  in  combination  with  the  rhombic-dodecahedron 
by  garnet  (29,  i  and  2).  Other  cubic  minerals  rep- 
resented on  the  colored  plates  are  zinc-blende,  tetra- 
hedrite,  and  sodalite.  The  special  hemihedral  forms 
of  iron-pyrites  and  of  zinc-blende  and  tetrahedrite 
will  be  mentioned  under  these  minerals. 


TETRAGONAL    SYSTEM 


19 


TETRAGONAL  SYSTEM 

Corresponding  to  the  octahedron  of  the  cubic  sys- 
tem, we  have  in  the  tetragonal  system  a  solid  also 
bounded  by  eight  equal  triangles;  but  here  the  tri- 
angles are  isosceles  and  not  equilateral,  two  of  their 
sides  being  equal  and  either  longer  or  shorter  than 


Fig.  14  Fig.  15 

Tetragonal  Bipyramids.* 


Fig.  16 

Tetragonal    Prism  with 
Basal  Planes. 


the  third.  This  form  is  a  double  pyramid  on  a  square 
base,  and  is  known  as  a  tetragonal  bipyramid  (Figs. 
14  and  15).  In  Fig.  14  the  vertical  axis  joining  the 
upper  and  lower  apices  of  the  pyramid  is  longer  than 
the  two  (equal)  horizontal  axes,  and  we  have  a  steep 
or  acute  pyramid.  In  Fig.  15  the  vertical  axis  is 
shorter,  and  the  pyramid  is  low  or  obtuse. 

*In  Fig.  14  the  vertical  axis  has  been  made  twice  the  length  of  tjie 
equal  horizontal  axes ;  and  in  Fig.  15  it  is  half  the  length  of  the  latter. 


20          THE   WORLD'S  MINERALS 

Here,  then,  we  see  that  there  may  be  a  variation  in 
the  angle  between  the  faces  of  the  crystal.  In  the 
cubic  system  the  angles  of  the  octahedron  are,  of 
course,  always  the  same  whatever  be  the  mineral 
crystallizing  in  this  form.  In  the  tetragonal  system, 
on  the  other  hand,  the  angle  between  the  faces  of  the 
pyramid  will  be  different  for  different  minerals,  but 
it  will  always  be  the  same  for  the  same  kind  of  min- 
eral. The  particular  value  of  this  angle  (from  which 
can  be  calculated  the  ratio  or  relative  lengths  of  the 
vertical  and  horizontal  axes)  is  fixed  for,  and  charac- 
teristic of,  each  kind  of  mineral.  Other  pyramids 
may,  however,  be  present  if  they  intersect  the  unit 
vertical  axis  in  simple  multiple  or  sub-multiple  dis- 
tances; and  in  this  way  we  may  have  a  combination 
of  steep  and  low  pyramids  on  the  same  crystal. 

Suppose  that  the  pyramid  faces  become  so  steeply 
inclined  that  they  intersect  the  vertical  axis  at  an  in- 
finite distance,  we  should  then  have  four  faces 
parallel  to  this  axis,  and  so  obtain  a  square  prism. 
Again,  we  may  imagine  the  pyramid  to  become  flat- 
ter and  flatter  until  finally  its  four  upper  faces  and 
its  four  lower  faces  coincide  in  one  plane  or  pair  of 
parallel  planes.  We  then  have  a  form  of  the  tetrag- 
onal system  known  as  the  basal  plane,  or,  taking  the 
two  parallel  planes  together,  as  the  basal  pinacoid. 
Neither  of  these  forms — the  square  prism  and  the 
basal  pinacoid — completely  encloses  space,  so  that  in 
crystals  they  can  exist  only  in  combination  with  other 
forms.  Fig.  16,  for  instance,  is  a  combination  of 
these  two  simple  forms,  the  ends  of  the  tetragonal 


ORTHORHOMBIC    SYSTEM  21 

prism  being  closed  by  the  pair  of  parallel  faces  of 
the  basal  pinacoid. 

Tetragonal  crystals  are  represented  on  the  colored 
plates  by  scheelite  (Plate  22,  Fig.  4)  in  simple  tetrag- 
onal bipyramids;  wulfenite  (22,  3)  and  cuprou- 
ranite  (24,  3)  in  flat  square  plates  with  a  large  de- 
velopment of  the  basal  pinacoid.  Combinations  of 
two  tetragonal  prisms  with  a  pyramid  are  shown  by 
zircon  (14,  4)  and  idocrase  (29,  4),  the  latter  show- 
ing also  the  basal  plane.  Other  tetragonal  minerals 
here  represented  are  copper-pyrites  (8,  i  and  2)  and 
cassiterite  (14,  3). 

ORTHORHOMBIC  SYSTEM 

Here,  again,  the  primary  form  is  a  solid  bounded 
by  eight  equal  triangles,  but  now  the  edges  of  the 
c 


Fig.  17  fte-  18 

Rhombic  Bipyramid.  Rhombic  Prism  with  Pinacoid. 

triangles  are  all  unequal.  This  form  is  called  a 
rhombic  bipyramid  (Fig.  17),  since  the  three  sec- 
tions of  it  made  by  planes,  each  passing  through  four 


22          THE   WORLD'S    MINERALS 

of  its  corners,  are  all  rhombs.  These  three  planes 
each  cut  the  solid  into  two  equal  and  similar  halves, 
and  they  are  therefore  planes  of  symmetry  (cor- 
responding to  the  planes  ABA'B',  ACA'C',  and 
BCB'C  of  the  regular  octahedron).  In  the  ortho- 
rhombic  system  these  are  the  only  possible  planes  of 
symmetry.  Other  simple  forms  are  rhombic  prisms 
and  pinacoids.  The  prisms  consist  of  four  faces 
parallel  to  one  or  other  of  the  three  axes,  and  they 
have  a  rhomb-shaped  cross-section.  The  three  pina- 
coids each  consist  of  a  pair  of  faces  parallel  to  two 
of  the  axes.  Fig.  18  shows  a  rhombic  prism  parallel 
to  the  axis  BB',  its  ends  being  closed  by  the  pinacoid 
parallel  to  the  axes  AA'  and  CC'. 

From  the  angles  between  the  faces  of  the  crystals 
it  is  possible  to  calculate  the  relative  lengths  of  the 
three  unequal  axes  AA',  BB',  and  CC.  These  values 
are  different  for  different  minerals,  but  constant  for 
crystals  of  the  same  mineral. 

Many  minerals  crystallize  in  this  system;  on  the 
colored  plates  examples  are  shown  of  orthorhombic 
crystals  of  sulphur,  mispickel,  marcasite,  atacamite, 
manganite,  cerusite,  aragonite,  barytes,  anglesite,  cel- 
estite,  olivine,  topaz,  andalusite,  staurolite,  and  col- 
umbite. 

MONOCLINIC  SYSTEM 

Here  we  have  an  oblique  angle  between  two  of 
the  three  axes,  and  this  system  is  consequently  often 
known  as  the  oblique  system.  The  crystals  possess 
only  one  plane  of  symmetry,  as  may  be  seen  from  the 


RHOMBOHEDRAL    SYSTEM  23 

figures  of  gypsum  (Plate  21,  Fig.  i  )and  augite  (27, 
i).  The  simple  forms  consist  of  no  more  than  two 
or  four  faces,  which  of  themselves  cannot  enclose 
space;  so  that  all  crystals  of  this  system  consist  of 
two  or  more  simple  forms  in  combination.  The  crys- 
tals of  lazulite  shown  in  Plate  24,  Fig.  2,  consist  of 
a  combination  of  two  monoclinic  hemi-pyramids; 
and  here,  since  the  oblique  angle  does  not  differ 
much  from  90°,  the  combined  form  has  quite  the  ap- 
pearance of  a  rhombic  pyramid. 

Other  common  minerals  crystallizing  in  this  sys- 
tem are  wolframite,  orthoclase,  hornblende,  diopside, 
epidote,  muscovite,  zinnwaldite,  clinochlore,  and 
sphene,  all  of  which  are  represented  on  the  plates. 

i 
ANORTHIC  SYSTEM 

Here  not  only  are  all  the  axes  of  unequal  lengths, 
but  they  are  all  inclined  at  oblique  angles.  There 
are  no  planes  or  axes  of  symmetry,  but  only  a  center 
of  symmetry.  Each  simple  form,  therefore,  consists 
only  of  a  pair  of  parallel  faces,  and  a  complete  crystal 
must  be  bounded  by  at  least  three  such  forms. 

Examples  are  given  by  microcline,  anorthite,  ky- 
anite,  and  axinite. 

RHOMBOHEDRAL  SYSTEM 

In  this  system  the  typical  or  primary  form  is  a 
rhombohedron — that  is,  a  solid  bounded  by  six  equal 
rhomb-shaped  faces.  This  form  possesses  a  certain 


24          THE   WORLD'S   MINERALS 

resemblance  to  the  cube.  If  we  stand  a  cube  up  on 
one  corner  with  one  of  its  four  triad  axes  (p.  12)  in  a 
vertical  position,  and  imagine  the  solid  to  be  drawn 
out  or  compressed  along  this  direction,  we  obtain  a 
rhombohedron.  An  acute  rhombohedron  so  pro- 
duced by  the  elongation  of  a  cube  is  shown  in  Fig. 


Fig.  19  Fig.  20  Fig.  21 

Acute  Rhombohedron.         Obtuse  Rhombohedron.      Hexagonal  Prism 

with  Basal  Pinacoid. 

19,  and  an  obtuse  rhombohedron  formed  by  the  com- 
pression of  a  cube  along  the  triad  axis  is  shown  in 
Fig.  20.  These  forms  exhibit  a  three-fold  arrange- 
ment of  their  faces  about  the  vertical  axis,  which  is 
thus  an  axis  of  triad  symmetry,  corresponding  to  one 
of  the  triad  axes  of  the  cube.  There  are  also  three 
vertical  planes  of  symmetry,  each  perpendicular  to 
a  face  of  the  rhombohedron,  and  coinciding  with  an 
edge.  All  the  edges  of  an  ideally  developed  rhom- 
bohedron are  equal  in  length  and  are  equally  inclined 
to  the  vertical  axis;  and  these  are  the  lines  which  in 


HEXAGONAL    SYSTEM  25 

this  system  of  crystals  are  taken  as  the  axes  of  ref- 
erence. 

If  we  imagine  a  rhombohedron  to  be  enormously 
elongated  in  the  direction  of  the  vertical  axis,  its  six 
faces  will  eventually  become  parallel  to  this  axis, 
and  a  hexagonal  prism  will  result.  If,  on  the  other 
hand,  the  rhombohedron  be  compressed  in  the  same 
direction,  the  limiting  form  will  consist  of  a  pair  of 
faces  perpendicular  to  the  vertical  axis;  this  form 
is  then  known  as  the  basal  pinacoid.  Fig.  21  shows 
a  combination  of  a  hexagonal  prism  and  the  basal 
pinacoid.  Another  simple  form  of  importance  in  the 
rhombohedral  system  is  that  known  as  the  scaleno- 
hedron;  this  is  a  solid  bounded  by  twelve  scalene  tri- 
angles, and  still  showing  a  three-fold  arrangement 
about  the  vertical  axis. 

Simple  rhombohedra  are  shown  by  the  minerals 
calcite  (Plate  16,  Figs.  2  and  3),  rhodochrosite  (17, 
3),  chalybite  (5,  3),  and  chabazite  (36,  2) ;  and  in 
combination  with  other  forms  by  pyrargyrite,  prous- 
tite,  quartz,  haematite,  corundum,  and  tourmaline. 

HEXAGONAL  SYSTEM 

This  system  has  many  points  in  common  with  the 
rhombohedral  system,  but  instead  of  showing  a  three- 
fold arrangement  of  faces  about  the  vertical  axis, 
there  is  a  six-fold  arrangement.  In  the  crystals  of 
apatite,  pyromorphite,  vanadinite,  and  beryl,  illus- 
trated on  the  colored  plates,  the  only  forms  present 
are  the  hexagonal  prism  and  the  basal  pinacoid,  ex- 


26          THE   WORLD'S    MINERALS 

actly  as  in  Fig.  21  of  the  rhombohedral  system.  In 
some  crystals,  however,  especially  those  of  beryl  and 
apatite,  many  more  faces  are  sometimes  present. 

i 

THE  HABIT  OF  CRYSTALS 

Apart  from  the  actual  degree  of  symmetry  which 
they  possess  and  the  system  to  which  they  belong, 
crystals  very  often  present  a  certain  kind  of  develop- 
ment or  "habit,"  and  this  may  often  be  a  character- 
istic feature  of  some  minerals.  Differences  in  habit 
result  from  the  unequal  development  of  the  different 
forms  or  faces  of  the  crystals.  The  prism  faces,  for 
instance,  may  be  greatly  extended,  and  we  then  have 
a  columnar  crystal,  which  is  said  to  be  prismatic  in 
habit,  as  in  crystals  of  beryl  (Plate  28,  Fig.  4),  ara- 
gonite  (28,  4),  andalusite  (31,  i  and  3),  tourmaline 
(33,  i),  etc.  If  the  prisms  are  relatively  longer  and 
thinner,  the  habit  may  be  described  as  long-prismatic, 
or  rod-like;  as  in  stibnite  (4,  2),  actinolite  (26,  2), 
epidote  (32,  i),  etc.  When  the  prisms  are  still  thin- 
ner, the  habit  becomes  acicular  or  needle-like;  as  in 
crocoite  (22,  i).  In  some  crystals  the  prisms  may  be 
so  very  long  and  slender  that  the  habit  is  capillary, 
or  hair-like;  this  is  an  extremely  characteristic  habit 
of  the  mineral  millerite  (sulphide  of  nickel),  which 
on  this  account  is  sometimes  known  as  hair-pyrites. 

If,  on  the  other  hand,  the  prism  faces  be  short  and 
the  basal  plane  largely  developed,  the  crystal  will  be 
plate-like,  or  tabular,  in  habit.  Examples  of  this 
are  shown  by  barytes  (Plate  20,  Fig.  i),  wulfenite 


INTERGROWTHS    OF    CRYSTALS    27 

(22,  3),  the  micas  (34,  1-3),  and  chlorites  (34,  4). 
This  tabular  habit  is  especially  characteristic  of  the 
micas,  the  crystals  of  which  often  have  the  form  of 
small  scales.  A  pyramidal  habit  is  usually  shown  by 
native  sulphur  (1,3),  scheelite  (22, 4) ,  etc.  Rock-salt 
(10,  i  and  2)  and  fluor-spar  (9,  i  and  2)  are  char- 
acterized by  their  cubic  habit,  while  diamond  (i,  i) 
and  magnetite  (14,  i)  most  frequently  exhibit  an 
octahedral  habit;  octahedra  of  rock-salt  and  fluor- 
spar and  cubes  of  diamond  and  magnetite  are,  how- 
ever, sometimes  found. 

INTERGROWTHS  OF  CRYSTALS 

Intergrowths  of  two  or  more  crystals  are  of  fre- 
quent occurrence.  Two  crystals  which  commenced 
their  growth  at  neighboring  centers  would  in  time 
fill  up  the  intervening  space;  they  would  then  co- 
alesce and  hinder  each  other's  growth.  The  two 
individuals,  having  commenced  their  growth  quite 
independently,  would  naturally  not  be  related  to  one 
another  in  any  definite  manner,  and  their  relative 
positions  would  be  quite  accidental.  Examples  of 
irregular  intergrowths  of  this  kind  are  to  be  seen 
on  almost  all  of  the  accompanying  plates.  Some- 
times, however,  there  may  be  a  parallel  grouping  of 
the  indivdual  crystals;  as  shown,  for  example,  by 
barytes  in  Plate  20,  Fig.  2,  and  celestite  in  Fig.  4  of 
the  same  plate. 

In  certain  cases,  however,  the  two  individuals  are 
grown  together  in  a  certain  definite  and  regular  man- 


28          THE    WORLD'S    MINERALS 

ner,  as  if  one  were  the  mirror-reflection  of  the  other. 
Such  a  grouping  is  called  a  twinned  crystal,  or  a 
twin,  and  the  plane  of  reflection  is  called  the  twin- 
plane.  One  portion  of  the  twin  can  be  brought  into 
a  position  identical  with  that  of  the  other  portion 
by  a  rotation  of  half  a  revolution  (180°)  about  an 
axis  perpendicular  to  the  twin-plane.  Twinned  crys- 
tals usually  show  re-entrant  angles  between  the  faces 
of  the  two  individuals,  and  very  often  this  is  errone- 
ously taken  as  an  indication  of  the  presence  of  twin- 
ning; but  it  must  be  remembered  that  in  any  acci- 
dental or  parallel  intergrowth  of  the  two  crystals 
there  would  also  be  re-entrant  angles  between  the 
faces. 

A  few  good  examples  of  twinned  crystals  are 
shown  on  the  colored  plates.  The  cross  of  staurolite 
(Plate  33,  Fig.  5)  clearly  shows  the  intergrowth  of 
two  prismatic  crystals  almost  at  right  angles  to  one 
another.  In  Plate  21,  Fig.  2,  is  shown  a  "swallow- 
tail" twin  of  the  gypsum.  Plate  9,  Fig.  i,  shows  in- 
terpenetrating twinned  cubes  of  fluor-spar  with  the 
corners  of  smaller  cubes  projecting  from  the  faces 
of  larger  cubes;  and  Plate  36,  Fig.  2,  very  similar 
groups  of  chabazite,  but  here  the  crystals  are  rhom- 
bohedra  with  very  nearly  the  shape  of  cubes.  Knee- 
shaped  twins  of  cassiterite  are  shown  in  Plate  14,  Fig. 
3 ;  and  in  marcasite  (Plate  7,  Fig.  i )  five  orthorhom- 
bic  crystals  are  twinned  together  to  produce  a  pen- 
tagonal form.  The  pseudo-hexagonal  prisms  of 
aragonite  (Plate  18,  Fig.  3)  are  also  the  result  of  the 
twinning  together  of  several  orthorhombic  crystals. 


AGGREGATION    OF    CRYSTALS      29 


FORMS  OF  AGGREGATION  OF  CRYSTALS 

Many  minerals,  instead  of  growing  as  single  and 
distinctly  developed  crystals,  give  rise  to  various, 
more  or  less,  accidental  shapes  and  various  kinds  of 
structure  or  texture,  due  to  the  crowding  together 
of  a  large  number  of  crystal  individuals.  Although 
this,  of  course,  complicates  the  study  of  the  crystals 
themselves,  yet  these  forms  of  aggregation  are  often 
extremely  characteristic  of  particular  minerals. 

When  the  material  consists  of  a  vast  number  of 
minute  crystalline  individuals  closely  crowded  to- 
gether, and  each  individual  is  developed  to  approxi- 
mately the  same  extent  in  all  directions,  we  have  a 
granular  crystalline  structure.  Here  the  crystalliza- 
tion of  the  material  had  clearly  started  simultane- 
ously at  a  large  number  of  neighboring  centers,  and 
all  the  intervening  spaces  soon  became  filled  up, 
forming  a  mass  of  crystalline  grains,  but  without  the 
development  of  any  crystal  faces.  Such  a  structure 
is  well  shown  by  statuary  marble  and  by  loaf-sugar; 
on  fractured  surfaces  of  these  materials  the  extent 
of  each  crystalline  individual  forming  the  mass  is 
readily  seen  when  the  bright  cleavage  surfaces  are 
examined  with  a  magnifying-glass.  If  the  crystal- 
line grains  are  so  small  that  they  cannot  be  dis- 
tinguished, except  under  the  high  powers  of  a  mi- 
croscope, the  structure  is  described  as  compact,  and 
the  mineral  is  said  to  be  massive.  Examples  of  this 
are  shown  on  the  colored  plates  by  jasper  (Plate  12, 


30          THE   WORLD'S    MINERALS 

Figs.  2  and  3),  hornstone  (12,  4),  pitchblende  (14, 
5),  turquoise  (24,  5),  nephrite  (26,  4),  lapis-lazuli 
(28,  2),  serpentine  (35,  i),  and  meerschaum  (35,  2). 
Masses  of  these  minerals  will,  of  course,  show  no 
external  crystalline  form;  and  if  they  present  any 
external  surface,  other  than  that  of  fracture,  this  will, 
as  a  rule,  be  rounded,  or  nodular. 

A  different  kind  of  structure  results  when  the  in- 
dividual crystals  of  the  aggregate  are  greatly  elon- 
gated in  one  direction.  If  the  crystals  are  of  an 
appreciable  size,  the  structure  may  be  said  to  be 
columnar,  while  if  they  are  finer  the  structure  is 
described  as  fibrous.  Now,  we  may  have  different 
kinds  of  fibrous  structure,  according  to  the  manner 
in  which  the  fibers  are  arranged  in  the  aggregate. 
They  may  have  a  parallel  arrangement,  as  in  ceru- 
site  (Plate  18,  Fig.  4),  gypsum  (21,  3)  and  crocido- 
lite  (26,  3).  When  the  fibers  are  very  fine  this 
parallel  fibrous  structure  gives  rise  to  a  silky  luster 
in  the  mineral,  as  is  to  be  seen  in  the  satin-spar 
variety  of  gypsum  and  in  crocidolite.  Again,  the 
fibers  may  be  matted  together  to  produce  a  felt-like 
aggregate;  as  in  the  mineral  known  as  mountain- 
leather,  or  mountain-cork.  Or,  again,  they  may  ra- 
diate from  a  center,  producing  star-like  groups.  The 
last  form  of  aggregation  is  especially  common  among 
minerals,  and  may  be  of  different  degrees  of  coarse- 
ness or  fineness;  for  example,  in  tourmaline  (Plate 
33>  FiS-  3)>  chessylite  (19,  i),  stibnite  (4,  i),  pyrolu- 
site  (15,  2),  wavellite  (24,  i),  natrolite  (37,  i  and 
2),  etc. 


AGGREGATION   OF   CRYSTALS      31 

Very  often  minerals  with  a  radially  fibrous  struc- 
ture present  on  their  free  surfaces  a  rounded  form. 
Many  varieties  of  such  external  rounded  forms  may 
be  distinguished,  and  these  are  sometimes  quite  char- 
acteristic of  certain  minerals.  A  globular  or  hemi- 
spherical form  is  invariably  shown  by  wavellite 
(Plate  24,  Fig.  i ;  here  the  hemispherical  aggregates 
have  been  broken  across).  Nodules  of  haematite  are 
very  often  kidney-shaped,  or  reniform  (13,  2).  In 
the  mineral  prehnite  the  rounded  forms  are  them- 
selves often  aggregated  like  a  bunch  of  grapes,  and 
on  this  account  this  form  of  aggregation  is  described 
as  botryoidal.  Mamillary  or  breast-like  surfaces  are 
shown  by  malachite  (19,4)  and  native  arsenic  (2,  i). 
Combined  with  a  radially  fibrous  structure,  there 
may  also  be  a  concentric  shelly  structure,  as  shown 
by  malachite  (19,  4)  and  aragonite  (18,  2).  In  the 
last  instance  the  concentric  shelly  structure  predom- 
inates ;  and  a  number  of  the  pea-like  forms  are  them- 
selves aggregated  to  give  a  pisolitic  structure. 

Again,  we  may  have  a  radial  arrangement  of  fibers, 
not  about  a  point,  but  around  a  line  or  axis;  and  the 
external  form  may  then  be  stalactitic  (e.g.  psilomel- 
ane,  15,  4),  or  coralloidal  (e.g.  aragonite,  18,  i). 
Such  a  coralloidal  form  is  especially  characteristic 
of  the  variety  of  aragonite  known  as  flos-ferri. 

Many  other  forms  of  aggregates  might  be  men- 
tioned— for  example,  mossy,  leafy,  wiry,  (native 
silver,  Plate  2,  Fig.  5),  dendritic  or  tree-like  (wollas- 
tonite,  27,  6) .  The  almond-like  form  of  agates  arises 
from  the  mineral  filling  a  rock-cavity  of  this  shape. 


32          THE    WORLD'S    MINERALS 


AMORPHOUS  MINERALS 

There  are  but  few  minerals  that  possess  no  indi- 
cations of  crystalline  structure.  These  are  of  the 
nature  of  glasses,  or  colloids;  and  the  best  example 
is  afforded  by  opal  (Plate  16,  Figs.  4  and  5).  Such 
minerals  present  a  smooth  and  glassy  fracture,  and 
usually  occur  as  fillings  in  rock-cavities ;  sometimes, 
however,  they  may  present  rounded  or  botryoidal 
external  surfaces. 

PSEUDOMORPHS 

When  a  mineral,  still  in  the  earth's  crust,  is  ex- 
posed to  conditions  other  than  those  that  prevailed  at 
the  time  of  its  origin  and  growth,  it  may  happen  that 
these  will  lead  to  its  destruction.  The  crystals  may 
be  re-dissolved,  and  their  material  carried  away  in 
solution  to  be  deposited  elsewhere;  or  chemical 
changes  may  take  place  and  a  new  generation  of 
minerals  be  produced.  There  are  thus  in  the  inor- 
ganic world,  just  as  in  the  organic,  periods  of  growth 
and  decay,  and  minerals  are  by  no  means  so  perma- 
nent as  might  be  imagined. 

A  crystal  of  iron-pyrites  (sulphide  of  iron)  em- 
bedded in  the  solid  rock  may  at  last  be  attacked  by 
percolating  surface  waters  containing  oxygen  in  so- 
lution; its  sulphur  would  be  removed  as  sulphuric 
acid  or  as  calcium  sulphate,  while  the  iron  would 
be  oxidized  and  take  up  water,  so  giving  rise  to  a 
new  growth  of  the  mineral  limonite  (hydroxide  of 


PSEUDOMORPHS  33 

iron).  The  space  formerly  occupied  by  the  cube 
of  iron-pyrites  is  now  taken  up  by  limonite,  a  mineral 
which  never  crystallizes  as  cubes,  or  indeed  in  any 
other  form.  We  have  then  a  pseudomorph  (or  false 
form)  of  limonite  after  iron-pyrites. 


CHAPTER  III 

THE  PHYSICAL  CHARACTERS  OF  MINERALS 

THE  physical  characters  of  minerals  afford  many 
extremely  interesting  subjects  for  study,  but  here  we 
can  touch  on  only  a  few  of  the  more  important  points. 
Of  especial  interest  is  the  fact  that  in  crystals  many 
of  the  physical  characters  vary  with  the  direction 
within  the  crystal.  For  instance,  light  travels 
through  a  crystal  with  different  velocities  according 
to  the  direction  of  transmission ;  in  other  words,  a  ray 
of  light  can  travel  more  quickly  along  some  paths 
within  a  crystal  than  along  others.  The  action  of 
crystals  on  light  presents  many  problems  of  extreme 
complexity,  which  can  be  studied  only  with  the  aid 
of  special  instruments.  Nevertheless,  the  color  of 
minerals  and  of  their  crystals  is  a  character  that  first 
attracts  attention;  so  that  this  part,  at  least,  of  the 
subject  must  be  dealt  with  here. 

COLORS  OF  MINERALS. 

The  colors  of  some  minerals  are  so  well  known 
that  they  are  used  as  descriptive  terms  in  defining 
color;  for  instance,  we  speak  of  gold-yellow  or  gold- 
en, silver-white,  emerald-green,  ruby-red,  sapphire- 
blue,  etc.  It  would,  indeed,  be  possible  to  draw  up 
a  complete  nomenclature  of  colors  by  comparison 

34 


COLORS    OF   MINERALS  35 

with  minerals,  for  these  exhibit  every  possible  range 
of  color.  The  recognition  of  minerals  by  their  color 
alone  is,  however,  very  uncertain,  and  a  wide  expe- 
rience is  necessary  before  such  a  knowledge  can  be 
of  much  practical  value. 

Many  species  of  minerals  may  themselves  exhibit 
a  wide  range  of  color,  so  that  one  and  the  same  kind 
of  mineral  may  be  very  different  in  its  appearance  in 
this  respect.  For  instance,  the  popular  idea  of  topaz 
is  a  gem-stone  of  a  sherry-yellow  color;  but  when 
quite  pure  and  free  from  coloring  matter  this  stone 
is  perfectly  colorless  and  water-clear;  or  again,  it 
may  be  pink,  bluish,  or  greenish.  In  the  same  way, 
the  mineral  corundum  is  colorless  when  quite  pure; 
but  other  crystals  may  range  through  all  the  colors 
of  the  rainbow — red  (ruby),  orange,  yellow  (orien- 
tal topaz),  green  (oriental  emerald),  blue  (sap- 
phire), and  violet  (oriental  amethyst),  and  when 
larger  amounts  of  impurities  are  present  we  have 
the  black  emery;  all  these  are  color- varieties  of  one 
and  the  same  kind  of  mineral.  Quartz,  fluor-spar,  and 
many  other  minerals  show  similar  ranges  of  color. 

In  the  instances  just  mentioned  the  crystals  are 
transparent  and  clear,  and  they  owe  their  color  to  the 
presence  of  very  small  amounts — often  mere  traces — 
of  coloring  matter  diffused  through  the  substance  it- 
self in  much  the  same  way  that  a  dye  is  dissolved  in 
water. 

In  other  cases  we  may  have  a  colorless  crystal  col- 
ored by  the  inclusion  of  particles  of  other  minerals, 
which  may  be  very  minute,  and  present  in  such  con- 


36          THE   WORLD'S   MINERALS 

siderable  numbers  as  to  make  the  crystal  opaque.  A 
good  instance  of  this  is  given  by  ferruginous  quartz 
(or  Eisenkiesel)  and  jasper,  which  owe  their  yellow 
and  red  colors  to  the  presence  of  yellow  and  red 
oxides  of  iron  in  relatively  large  amounts. 

In  the  two  cases  so  far  mentioned  the  color  shown 
by  the  crystals  is  not  the  color  of  the  pure  mineral — 
which  of  itself  is  often  white  or  colorless — but  is  due 
to  the  presence  of  some  coloring  matter.  The  true 
color  may  be  determined  by  crushing  a  fragment  of 
the  mineral  on  a  sheet  of  white  paper,  or  by  rubbing 
a  piece  on  unglazed  porcelain.  This  is  known  as  the 
streak  of  the  mineral,  and  the  color  of  the  streak  is 
a  much  more  definite  character  of  a  mineral  than 
the  color  seen  in  bulk. 

Other  minerals  possess  a  color  of  their  own,  and 
for  these  the  color  of  the  streak  is  the  same,  or  nearly 
the  same,  as  the  color  shown  by  the  mineral  in  bulk. 
The  colors  of  gold  and  copper,  for  instance,  are 
inherent  in  these  substances;  it  is,  however,  to  be 
remembered  that  native  gold  may  be  paler  in  color 
when  alloyed  with  much  silver,  and  that  native  cop- 
per is  often  black  or  green  on  the  surface,  owing  to 
alteration.  Cinnabar  (Plate  6,  Figs.  2  and  3),  orpi- 
ment  (4,  3),  malachite  (19,  3  and  4),  and  chessylite 
(19,  i  and  2),  again,  are  minerals  with  colors  of  their 
own,  respectively  red,  yellow,  green,  and  blue,  which 
are  the  same  also  in  the  streaks.  The  different  ores  of 
iron  can  be  readily  distinguished  by  the  colors  of 
their  streaks;  that  of  magnetite  is  black,  haematite 
red,  limonite  brown,  and  chalybite  white. 


TRANSPARENCY    AND    LUSTER     37 

The  colors  displayed  by  some  minerals  are  of  a 
totally  different  nature ;  they  are  not  in  the  minerals 
themselves,  but  are  produced  by  the  action  of  certain 
structures  in  the  mineral  on  white  light.  The  differ- 
ent colored  rays  of  which  white  light  is  composed 
are  acted  upon  in  such  a  way  that  certain  colors  are 
eliminated,  while  others  remain  as  flashing  rainbow 
colors.  The  interference-colors  so  produced  by  pre- 
cious opal  and  labradorite  are  of  the  same  nature  as 
those  shown  by  a  soap-bubble  or  by  a  film  of  oil 
on  water.  Of  the  same  nature  also  are  the  iridescent 
colors  shown  by  the  tarnished  surfaces  of  some  min- 
erals, or  reflected  from  cracks  in  the  interior  of 
crystals. 

Still  another  color  effect  of  crystals,  which  may  be 
mentioned  here,  is  that  known  as  pleochroism  or 
dichroism.  Many  colored  crystals  are  of  a  different 
color  according  to  the  direction  through  which  they 
are  viewed.  A  crystal  of  the  mineral  cordierite  (or 
dichroite),  for  instance,  is  dark-blue,  light-blue,  or 
straw-yellow,  when  viewed  through  in  three  direc- 
tions at  right  angles.  This  character,  which  can 
be  most  conveniently  examined  with  a  little  instru- 
ment called  a  dichroscope,  affords  a  ready  means  of 
distinguishing  many  kinds  of  colored  gem-stones. 

TRANSPARENCY  AND  LUSTER 

Some  minerals  are  always  opaque,  as,  for  example, 
the  metals  and  most  of  the  metallic  ores.  Others — 
e.g.  quartz — may  vary  from  perfect  transparency  to 


38          THE   WORLD'S    MINERALS 

complete  opacity,  an  intermediate  degree  being  de- 
scribed as  translucency ;  but  in  such  cases  the  speci- 
mens which  are  opaque  in  the  mass  will  show  light 
through  the  edges  of  thin  splinters. 

The  luster  of  minerals  varies  not  only  in  intensity, 
but  also  in  kind,  and  the  kind  of  luster  is  often  a  very 
characteristic  feature  of  different  minerals.  The 
metallic  luster  of  metals  and  many  metallic  ores  is, 
for  instance,  quite  different  in  character  from  the 
glassy  (or  vitreous)  luster  of  quartz ;  and  the  peculiar 
adamantine  luster  of  diamond  enables  this  mineral 
to  be  recognized  at  a  glance.  The  luster  of  other 
minerals  may  be  waxy,  greasy,  resinous,  silky,  etc. 

The  physical  characters  so  far  enumerated  depend 
on  the  action  of  crystals  on  light.  On  the  other  hand, 
some  minerals  are  themselves  acted  upon  by  light; 
on  exposure  they  may  lose  their  color,  transparency, 
and  luster.  A  striking  instance  of  this  is  afforded 
by  the  mineral  proustite  (Plate  8,  Fig.  6),  which  is 
ruby-red,  transparent,  and  has  a  brilliant  adamantine 
luster;  but  on  exposure  to  light  it  soon  becomes  black, 
opaque,  and  dull.  The  transparent,  aurora-red  crys- 
tals of  realgar  (Plate  4,  Fig.  4)  soon  fall  to  a  yellow 
powder  when  exposed  to  light.  In  collections,  such 
minerals  must,  therefore,  be  protected  from  the  light. 

SPECIFIC  GRAVITY 

After  color  and  luster,  the  character  which  next 
attracts  attention  is  that  of  heaviness.  It  is  soon  no- 
ticed that  some  minerals  are  heavier  than  others,  and 


SPECIFIC   GRAVITY 


in  this  respect  there  are  indeed  very  wide  differences. 
The  extremes  of  specific  gravity  range  from  1.05  in 
amber  to  23  in  iridium  ;  that  is,  amber  is  only  slightly 
heavier  than  an  equal  volume  of  water,  while  irid- 
ium is  twenty-  three  times  as  heavy.  The  following 
table  illustrates  how  various  common  minerals  differ 
in  their  specific  gravity. 


Amber 1.05 

Borax. 1.7 

Sulphur 2.1 

Rock-salt 2.15 

Gypsum 2.3 

Orthoclase. . .  2.56 

Quartz 2.65 

Calcite 2.72 

Muscovite. . .  2.9 

Fluor-spar. . .  3.2 


Jadeite 

Diamond... 

Chessylite . . 
Corundum. . 

Barytes 

Pyrolusite . . 
Iron-pyrites. 

Arsenic 

Mispickel. . . 
Cerusite. . 


3.33  Cassiterite. . .  7.0 

3.52      Galena 7-5 

3.8        Cinnabar 8.1 

4.0        Copper 8.85 

4.5        Bismuth 9-75 

48        Silver 10.6 

5.0        Lead 11.4 

5.7        Mercury 13.6 

6.0  Platinum....  17.0 

6.5        Gold 19-0 


The  specific  gravity  can  be  determined  with  ac- 
curacy, and  its  value  expressed  in  definite  numbers; 
and  being  different  for  different  minerals,  it  is  an 
extremely  valuable  character  of  determinative  value. 
With  a  little  practise,  it  is  possible  by  simply  hand- 
ling a  specimen  to  gain  some  idea  of  the  approximate 
value  of  its  specific  gravity,  and  so  to  distinguish 
between  minerals  which  may  be  alike  in  general  ap- 
pearance. 

The  methods  available  for  the  accurate  determi- 
nation of  this  most  important  character  are  explained 
in  the  text-books  on  physics;  they  depend  on  the  prin- 
ciple of  determining  the  weight  of  water  displaced 
by  the  body,  and  so  finding  the  ratio  between  the 
weight  of  the  body  and  the  weight  of  an  equal  volume 


40          THE   WORLD'S   MINERALS 

of  water.  A  specially  quick  and  ready  method,  par- 
ticularly suitable  for  determining  the  specific  gravity 
of  minerals,  is  that  given  by  the  use  of  heavy  liquids. 
The  liquid  compound  methylene  iodide  has  a  specific 
gravity  of  3.33  (i.e.  it  is  more  than  three  times  heav- 
ier than  water),  and  is  miscible  in  all  proportions 
with  benzole  (sp.  gr.  0.89).  A  cheaper*  heavy 
liquid  is  bromoform,  with  the  lower  specific  gravity 
of  2.8. 

If  fragments  of  the  minerals  named  in  the  above 
table  of  specific  gravities  be  dropped  into  methylene 
iodide,  fluor-spar  and  all  those  of  lower  specific 
gravity  will  float  on  the  surface  of  the  liquid,  while 
diamond  and  all  of  higher  specific  gravity  will  sink 
to  the  bottom;  jadeite,  with  a  specific  gravity  exactly 
the  same  as  that  of  the  liquid,  will,  however,  remain 
suspended  in  the  liquid,  neither  floating  nor  sinking. 
By  the  addition  of  benzole  to  the  methylene  iodide 
the  specific  gravity  of  the  liquid  may  be  reduced  to 
any  desired  amount;  so  that  any  of  the  lighter  min- 
erals will  remain  suspended. 

Selecting  a  series  of  known  minerals  of  known 
specific  gravity  for  use  as  indicators  in  the  heavy 
fluid,  we  shall,  by  comparison,  be  able  to  determine 
the  specific  gravity  of  a  fragment  of  an  unknown 
mineral,  and  this  will  be  of  great  assistance  in  de- 
termining the  kind  of  mineral.  This,  of  course,  can 

*Methylene  iodide  costs  about  80  cents  per  ounce,  and  bromoform 
about  20  cents  per  ounce.  Methylene  iodide  darkens  on  exposure  to 
light,  and  it  should,  therefore,  be  kept  covered;  any  depth  of  color  it 
may  acquire  can  be  removed  by  allowing  bits  of  metallic  copper  to 
remain  in  the  liquid. 


FRACTURE  AND  CLEAVAGE    41 

only  be  done  when  the  specific  gravity  of  the  mineral 
is  less  than  3.33;  but  still,  it  is  often  of  great  help  to 
learn  that  the  specific  gravity  of  an  unknown  mineral 
is  greater  than  this.  For  instance,  faceted  gem- 
stones  of  similar  appearance  can  often  be  readily  dis- 
tinguished by  simply  dropping  them  into  the  heavy 
liquid.  Thus,  if  we  wish  to  distinguish  between 
quartz  (sp.  gr.  2.65)  and  topaz  (sp.  gr.  3.5),  it  will 
at  once  be  seen  that  the  former  floats  and  the  latter 
sinks. 

THE  FRACTURE  AND  CLEAVAGE  OF  MINERALS 

When  minerals  or  crystals  are  broken  their  sur- 
faces of  fracture  present  many  points  of  difference, 
and  these  differences  are  important  aids  in  distin- 
guishing minerals  of  different  kinds.  The  crystals 
of  some  minerals  possess  a  special  kind  of  fracture 
known  as  cleavage,  it  being  possible  to  split  or  cleave 
them  along  plane  surfaces  parallel  to  certain  faces 
of  the  crystal.  Thus  crystals  of  rock-salt  or  galena, 
whether  they  be  cubes  or  of  any  other  form,  can  be 
readily  split  parallel  to  the  faces  of  the  cube.  Cubes 
of  fluor-spar  cannot  be  split  in  these  directions,  but 
only  parallel  to  the  faces  of  the  octahedron.  Dia- 
mond also  possesses  a  perfect  octahedral  cleavage, 
while  zinc-blende  cleaves  parallel  to  the  faces  of  the 
rhombic-dodecahedron.  These  cleavages  may  be  de- 
veloped parallel  not  only  to  one  face  of  these  simple 
forms,  but  parallel  to  all;  since  the  internal  crystal- 
line structure  of  the  material  possesses  the  same  de- 
gree of  symmetry  as  the  external  form  of  the  crystal. 


42          THE    WORLD'S    MINERALS 

Thus,  since  a  cube  is  bounded  by  three  pairs  of  paral- 
lel equivalent  faces,  there  must  be  in  rock-salt  and 
galena  three  directions  of  cleavage  at  right  angles  to 
one  another.  In  the  same  way,  fluor-spar  and  dia- 
mond each  have  four  directions  of  cleavage  parallel 
to  the  four  pairs  of  parallel  faces  of  the  octahedron; 
and  in  zinc-blende  there  are  six. 

In  calcite  there  are  three  directions  of  perfect 
cleavage  parallel  to  the  three  pairs  of  parallel  faces 
of  the  primary  rhombohedron.  In  crystals  belonging 
to  crystal-systems  other  than  the  cubic,  there  may  be 
only  one  plane  direction  of  cleavage,  since  there  can 
be  no  symmetrical  repetition  of  this  direction  within 
the  crystal.  Thus  the  micas  possess  one  very  perfect 
cleavage  parallel  to  the  basal  plane,  and  gypsum  one 
parallel  to  the  single  plane  of  symmetry. 

The  cleavage  of  a  crystal  is  an  expression  of  a 
minimum  of  cohesion  in  a  certain  direction  in  the 
material  of  the  crystal;  and  it  is  important  to  remem- 
ber that  the  crystal  can  be  split  up  indefinitely  along 
its  direction  of  cleavage.  Taking,  for  example,  a 
crystal  of  mica,  we  can  keep  splitting  off  thin  leaves 
until  the  whole  crystal  is  separated  into  flakes,  and 
these  flakes  can  still  further  be  subdivided  with  a 
knife-edge  along  the  same  direction  of  cleavage. 
With  the  harder  minerals  the  best  way  of  producing 
the  cleavage  is  to  place  a  knife-edge  on  the  crystal 
parallel  to  the  particular  crystal-face,  and  to  strike 
the  back  of  the  knife  with  a  sharp  blow  from  a 
hammer. 

The  cleavages  of  minerals  differ  not  only  in  di- 


HARDNESS  43 


rection  and  the  number  of  directions,  but  also  in  their 
perfection.  Some  minerals  cleave  more  readily  than 
others,  and  their  surfaces  of  separation  are  much 
smoother  and  brighter.  In  other  minerals  the  cleav- 
age may  be  so  poor  that  it  is  scarcely  noticeable. 
Here  the  kind  of  fracture  is  often  of  importance. 
Quartz  and  opal,  for  example,  break  with  a  smooth 
and  glassy  conchoidal  fracture,  the  surfaces  being 
rounded  and  having  curved  lines  like  the  lines  of 
growth  on  the  surface  of  a  bivalve  shell.  Other 
kinds  of  fracture  are  described  as  sub-conchoidal, 
splintery  (e.g.  nephrite),  hackly  (e.g.  copper),  etc. 

HARDNESS 

By  the  hardness  of  a  mineral  is  meant  its  capability 
of  scratching  other  substances.  Some  minerals  are 
so  soft  that  they  can  be  scratched  by  the  finger-nail, 
others  can  be  scratched  by  a  knife  or  file,  and  others 
again  are  still  harder.  For  expressing  degrees  of 
hardness,  mineralogists  make  use  of  the  following 
ten  minerals  as  a  scale  of  hardness,  ranging  succes- 
sively from  talc,  the  softest,  to  diamond,  the  hardest. 

1.  Talc  6.  Felspar 

2.  Gypsum  7.  Quartz 

3.  Calcite  8.  Topaz 

4.  Fluor-spar  9.  Corundum 

5.  Apatite  10.  Diamond 

Nos.  i  and  2  on  the  scale  can  be  scratched  by  the 
finger-nail;  Nos.  1-6  by  a  knife,  though  No.  6  only 
with  difficulty.  No.  6  will  scratch  ordinary  win- 


44          THE    WORLD'S    MINERALS 

dow-glass,  but  not  so  easily  as  will  No.  7.  Tests 
such  as  these  will  give  a  first  rough  idea  of  the  hard- 
ness of  an  unknown  mineral.  To  determine  its 
hardness  on  the  scale,  we  must  find  the  scale-mineral 
that  it  can  only  just  scratch,  and  the  lowest  one  that 
it  can  be  scratched  by.  Thus,  if  a  mineral  scratches 
calcite  about  as  easily  as  it  can  itself  be  scratched 
by  fluor-spar,  its  hardness  may  be  expressed  as  3^2. 
In  applying  this  test  it  is  best  to  rub  a  sharp  corner 
of  the  scratching  mineral  across  a  smooth  surface 
of  the  mineral  to  be  scratched;  the  powder  produced 
should  be  wiped  off,  and  the  surface  carefully  ex- 
amined with  a  lens,  to  make  sure  whether  a  scratch 
has  really  been  produced  or  only  powder  rubbed  off 
the  corner  of  the  scratching  mineral.  In  selecting  a 
smooth  surface,  such  as  a  cleavage  surface  or  a  crys- 
tal face,  care  must  be  taken  that  the  specimen  is  not 
damaged  or  a  good  crystal  spoilt.  Nothing  looks 
worse  in  a  collection  of  minerals  than  a  scratched 
and  damaged  specimen.  Earthy  or  loose  aggregates 
of  crystals  do  not,  of  course,  show  by  scratching  the 
true  hardness  of  a  mineral. 

MAGNETIC  AND  ELECTRICAL  PROPERTIES 

Magnetite  being  the  only  strongly  magnetic  min- 
eral, mention  of  this  property  may  be  deferred  until 
we  come  to  a  description  of  the  mineral  itself.  Most 
minerals,  except  metals  and  metallic  ores  (which  are 
good  conductors),  acquire  a  charge  of  electricity 
when  rubbed ;  sulphur  and  amber  so  acquire  a  nega- 


TOUCH,    TASTE,    AND    SMELL        45 

tive  charge,  and  most  gem-stones  a  positive  charge. 
Some  few  acquire  an  electrical  charge  when  heated 
(see  tourmaline). 

TOUCH,  TASTE,  AND  SMELL 

Some  minerals  are  greasy  or  soapy  to  the  touch — 
for  example,  talc,  which  for  this  reason  is  sometimes 
called  soapstone;  others  have  a  rough,  harsh  feel. 
A  few  minerals  which  are  soluble  in  water  possess  a 
characteristic  taste — for  example,  salt  and  epsom- 
salts.  A  few  others  have  a  characteristic  smell — for 
example,  the  bituminous  odor  of  asphaltum.  Clays 
when  breathed  upon  have  an  earthy  odor.  Iron- 
pyrites  and  mispickel,  when  struck,  emit,  respective- 
ly, a  sulphurous  and  a  garlic-like  odor. 


CHAPTER  IV 

THE  CHEMICAL  COMPOSITION  AND  CLASSI- 
FICATION OF  MINERALS 

MINERALS  being  chemical  elements  and  compounds, 
it  is  necessary  in  order  properly  to  understand  them 
to  have  some  knowledge  of  chemistry.  Of  the  eighty 
different  kinds  of  elementary  matter  known  to  chem- 
ists, all  are  found  in  minerals;  and  for  the  majority 
of  them,  minerals  are  the  only  source.  All  the  inor- 
ganic products  prepared  by  the  manufacturing  chem- 
ist are  derived  directly  from  minerals.  The  more 
common  chemical  elements  (forty-one  in  number) 
which  are  present  as  essential  constituents  of  the 
minerals  described  in  this  book  are  given  in  the  fol- 
lowing table,  together  with  their  chemical  symbols* 
and  their  atomic  or  combining  weights.  Several 
other  rarer  elements  may  also  be  present  in  these 
minerals;  for  example,  pitchblende  contains  the  ele- 
ments radium,  helium,  and  argon,  and  zinc-blende 
often  contains  the  elements  cadmium,  gallium,  and 
indium.  It  will  be  noticed  that  the  abundant  ele- 
ment nitrogen  is  not  represented  in  the  table;  the 
explanation  of  this  is  that  nitrates  and  ammonium 

*When  these  are  derived  from  the  Latin  name  of  the  element  this 
name  is  added  in  parentheses. 

46 


CHEMICAL    COMPOSITION 


salts,  being  all  soluble  in  water,  are  of  rare  occur- 
rence as  minerals. 


Atomic 

Symbol  weight 

Aluminium Al  27 

Antimony  (Stibium)  Sb  120 

Arsenic As  75 

Barium Ba  137 

Beryllium Be  9.1 

Bismuth Bi  208 

Boron B  n 

Calcium Ca  40 

Carbon C  12 

Chlorine Cl  35-5 

Chromium Cr  52 

Cobalt Co  59 

Copper   (Cuprum)..  Cu  63.5 

Fluorine F  19 

Gold  (Aurum) Au  197 

Hydrogen H  I 

Iron  (Ferrum) Fe  56 

Lead  (Plumbum)  ...  Pb  207 

Lithium Li  7 

Magnesium Mg  24 

Manganese Mn  55 


Atomic 

Symbol  weight 
Mercury  (Hydrargy- 
rum)   Hg  200 

Molybdenum Mo  96 

Niobium Nb  93.5 

Nickel Ni  58.7 

Oxygen O  16 

Phosphorus P  31 

Platinum Pt  195 

Potassium  (Kalium)  K  39 

Silicon Si  28 

Silver  (Argentum)..  Ag  108 

Sodium  (Natrium). .  Na  23 

Strontium Sr  87.6 

Sulphur S  32 

Tin  (Stannum) Sn  119 

Titanium Ti  48 

Tungsten  (Wolfram)  W  184 

Uranium U       ,  238.5 

Vanadium V  51.2 

Zinc Zn  65.4 

Zirconium Zr  90.6 


Of  the  elements  enumerated  above  only  four — 
namely,  oxygen,  hydrogen,  fluorine,  and  chlorine — 
are  gases  when  in  their  free  state;  only  one  is  liquid 
— namely,  mercury;  and  all  the  others  are  solid  at 
the  ordinary  conditions  of  temperature  and  pressure. 
The  gases,  together  with  carbon,  boron,  silicon,  sul- 
phur, and  phosphorus,  are  classed  as  non-metallic 
elements,  all  the  others  being  metals. 

The  chemical  compounds  met  with  as  minerals 
consist,  in  most  instances,*  of  a  combination  of  one 
or  more  metallic  elements  with  one  or  more  non- 

*A  notable  exception  is  the  common  mineral  quartz. 


48          THE    WORLD'S    MINERALS 

metallic  elements.  These  are  combined  together  in 
definite  proportions.  For  example,  iron-pyrites  con- 
sists of  a  combination  of  one  atom  of  iron  with  two 
atoms  of  sulphur — that  is,  56  parts  by  weight  of  iron 
with  32  X  2  =  64  parts  by  weight  of  sulphur.  The 
actual  weights  (grams  or  tons)  is  immaterial,  since 
the  atomic  weights  or  combining  weights  as  given  in 
the  above  table  are  only  ratios.  The  chemical  sym- 
bol, or  formula,  for  iron-pyrites  is  then  FeS2;  this  not 
only  expresses  that  the  compound  is  disulphide  of 
iron,  but  it  also  enables  us  to  calculate  the  percentage 
weight  of  each  constituent — namely,  46.6  per  cent, 
of  iron  and  53.4  per  cent,  of  sulphur.  Again,  calcite 
is  a  combination  of  40  parts  by  weight  of  calcium, 
with  12  parts  of  carbon,  and  16X3  =  48  parts  of 
oxygen,  the  formula  being  CaCOs,  which  expressed 
in  words  is  carbonate  of  calcium. 

In  order  fully  to  determine  the  nature  of  a  mineral, 
it  is  necessary  to  analyze  it  by  the  ordinary  methods 
of  chemical  analysis;  and  even  for  purposes  of  identi- 
fication it  is  often  necessary  to  apply  some  simple 
chemical  tests.  Mineral  carbonates,  for  example,  can 
always  be  recognized  by  the  fact  that  they  effervesce 
in  acids.  Sulphur  is  readily  tested  for  by  heating  the 
powdered  mineral  with  sodium  carbonate  before  the 
blowpipe  on  charcoal,  and  moistening  the  fused  mass 
with  water  on  a  silver  coin;  if  sulphur  is  present  a 
characteristic  black  stain  will  be  produced  on  the 
silver.  At  the  same  time,  if  a  heavy  metal  is  present 
in  the  mineral,  a  bead  of  the  metal  will  be  formed 
on  the  charcoal. 


CLASSIFICATION    OF    MINERALS    49 

Two  very  important  principles  relating  to  the 
chemistry  of  minerals  are  those  of  polymorphism 
(including  dimorphism)  and  isomorphism.  Several 
cases  are  known  in  which  totally  distinct  minerals 
are  identical  in  chemical  composition.  For  example, 
diamond  and  graphite  both  consist  simply  of  the  ele- 
ment carbon ;  but  they  consist  of  carbon  crystallized 
in  different  forms,  so  that,  although  chemically  alike, 
they  are  quite  distinct,  or  dimorphous,  substances. 
Again,  calcium  carbonate  (CaCOs)  crystallizes  in 
nature  either  as  calcite  or  as  aragonite,  which  also 
are  two  quite  distinct  minerals.  The  most  remark- 
able case,  however,  is  that  of  the  compound  titanium 
dioxide,  which  is  met  with  in  nature  as  the  three 
minerals  rutile,  anatase,  and  brookite;  these,  though 
chemically  identical,  are  distinct  in  the  form  of  their 
crystals  and  in  all  their  physical  characters.  Another 
instance  of  trimorphism  is  given  by  the  minerals  ky- 
anite,  andalusite,  and  sillimanite  (see  Chapter  XII). 

On  the  other  hand,  we  may  have  minerals  of  dif- 
ferent chemical  composition  appearing  in  crystals 
that  are  almost  identical  in  form;  such  minerals  are 
said  to  be  isomorphous.  Of  this  there  are  numerous 
examples  amongst  minerals,  one  of  the  best  being 
that  given  by  the  group  of  rhombohedral  carbonates. 
The  carbonates  of  the  chemically  related  elements 
calcium,  magnesium,  iron,  zinc,  and  manganese  all 
crystallize  in  rhombohedra  with  very  nearly  the  same 
angles  between  their  faces.  But  not  only  this,  they 
are  capable  of  so  intimately  growing  together  that 
they  may  all  help  to  build  up  one  and  the  same  crystal. 


50          THE    WORLD'S    MINERALS 

The  tiny  crystalline  bricks  of  these  different  sub- 
stances are  so  nearly  alike  in  their  shape  that  they 
will  all  fit  into  the  same  structure.  We  then  have 
what  is  called  a  mixed  crystal,  consisting  of  two  or 
more  of  these  carbonates  mixed  together  in  variable 
proportions.  In  such  cases  the  chemical  composition 
of  the  crystal  cannot  be  fully  expressed  by  a  simple 
chemical  formula. 

The  classification  adopted  in  the  descriptive  por- 
tion of  this  book  is  a  strictly  chemical  classification, 
on  the  same  lines  as  in  the  modern  treatises  on  scien- 
tific mineralogy.  In  the  first  group  are  placed  the 
chemical  elements;  in  the  next  three  the  sulphides, 
haloids,  and  oxides,  respectively;  and  in  succeeding 
groups  the  large  number  of  oxygen-salts,  these  being 
subdivided  according  to  the  acid  which  enters  into 
combination  with  a  metal  to  form  the  various  salts, 
such  as  carbonates,  sulphates,  phosphates,  silicates, 
etc.*  Within  each  of  these  groups,  minerals  which 
are  isomorphous  with  one  another  are  placed  to- 
gether. In  an  appendix  a  few  organic  substances, 
which,  strictly  speaking,  are  not  definite  mineral 
species,  are  brought  together. 

Other  systems  of  classification  are  of  course  pos- 
sible. For  example,  in  the  eighteenth  century,  before 
the  development  of  modern  chemistry,  natural  his- 
tory systems  of  classification  were  in  vogue.  These 
took  account  only  of  the  external  characters  of  min- 
erals, and  naturally  lead  to  very  deceptive  results; 

*An  enumeration  of  the  minerals  placed  under  each  of  these  headings 
will  be  found  in  the  List  of  Plates,  p.  ix. 


CLASSIFICATION    OF    MINERALS    51 

since  totally  distinct  minerals  may  often  closely  re- 
semble one  another  in  their  general  appearance  and 
color.  Another  method  of  classification,  and  one 
which  is  often  used  at  the  present  day,  is  to  group 
minerals  according  to  the  metals  they  contain.  But 
this  system,  though  of  convenience  to  the  practical 
miner,  is  not  altogether  satisfactory,  since,  unless 
duplication  is  to  occur,  certain  minerals  must  be  arbi- 
trarily allocated  to  one  group  or  another. 

As  an  example  of  such  a  classification,  based  on 
economic  lines,  the  following  may  be  given.  This 
will,  at  the  same  time,  give  some  idea  of  the  practical 
uses  of  the  minerals  described  in  the  present  volume. 
A  certain  amount  of  duplication  is  here  unavoidable ; 
for  example,  mispickel  appears  with  the  ores  of  iron, 
but  being  of  more  importance  as  an  ore  of  arsenic, 
it  also  finds  a  place  under  this  metal.  Linarite  comes 
under  both  copper  and  lead.  Again,  quartz  appears 
amongst  the  precious  stones,  the  rock-forming  min- 
erals, and  the  abrasives.  On  the  other  hand,  several 
interesting,  though  commercially  valueless,  minerals, 
which  will  be  described  farther  on,  do  not  fall  into 
this  classification. 

METALLIC  ORES 

GOLD:     Native  gold. 

SILVER:     Native  silver,  Pyrargyrite,  Proustite,  Tetrahedrite. 

COPPER:    Native  copper,  Copper-pyrites,  Tetrahedrite,  Atacamite,  Ches- 

sylite,  Malachite,  Linarite,  Cuprouranite. 
PLATINUM  :     Native  platinum. 
LEAD:    Galena,  Cerusite,  Anglesite,  Linarite,  Crocoite,  Wulfenite,  Pyro- 

morphite,  Vanadinite. 
ZINC:     Zinc-blende,  Calamine. 


52          THE   WORLD'S    MINERALS 

IRON:    Iron-pyrites,  Marcasite,  Pyrrhotite,  Mispickel,  Haematite,  Mag- 
netite, Limonite,  Chalybite,  Vivianite. 

MANGANESE:    Manganite,  Pyrolusite,  Psilomelane,  Rhodochrosite. 
NICKEL  :    Niccolite. 
COBALT:     Smaltite,  Erythrite. 
MERCURY  :    Cinnabar. 
TIN  :    Cassiterite. 

TUNGSTEN:    Wolframite,  Scheelite. 
URANIUM:    Pitchblende,  Cuprouranite. 
VANADIUM  :    Vanadinite. 
CHROMIUM  :    Crocoite. 
MOLYBDENUM  :     Molybdenite,  Wulfenite. 
ARSENIC:     Native  arsenic,  Realgar,  Orpiment,  Mispickel. 
ANTIMONY:     Native  antimony,  Stibnite. 
BISMUTH  :    Native  bismuth. 

PRECIOUS  STONES  (GEM-MINERALS) 

Diamond,  Corundum,  Opal,  Quartz  (Amethyst,  Smoky-quartz,  Rock- 
crystal,  Cat's-eye,  Rose-quartz,  Agate,  Jasper),  Zircon,  Turquoise,  Beryl, 
Garnet,  Olivine,  Idocrase,  Topaz,  Epidote,  Tourmaline,  Amber. 

ROCK-FORMING  MINERALS 

Quartz  group,  Felspar  group,  Amphibole  group,  Pyroxene  group, 
Sodalite,  Leucite,  Olivine,  Idocrase,  Mica  and  Chlorite  groups,  Serpen- 
tine, Talc. 

SPARRY  MINERALS   (SPARS) 

Fluor-spar,  Calcite  (calc-spar),  Aragonite,  Barytes  (heavy-spar), 
Celestite,  Gypsum. 

ABRASIVES 

Diamond,  Corundum  (emery),  Quartz,  Garnet. 

SALTS  (SOLUBLE  IN  WATER) 
Rock-salt. 

INFLAMMABLES 

Organic  substances  (Amber,  Asphaltum,  Coal,  etc.),  Graphite,  Dia- 
mond, Sulphur. 


CHAPTER  V 

THE  NATIVE  ELEMENTS 

ALTHOUGH  some  eighty  different  kinds  of  elementary 
matter  have  been  identified  by  chemists  in  the  ma- 
terials of  the  earth's  crust,  yet  only  few  of  these  are 
found  in  a  free  state  in  nature.  The  gaseous  ele- 
ments, oxygen  and  nitrogen,  which  form  the  bulk  of 
our  atmosphere,  can  scarcely  be  regarded  as  min- 
erals; though  the  former,  when  in  chemical  combi- 
nation, is  present  in  a  large  number  of  minerals.  The 
chemical  elements  may  be  roughly  divided  into  two 
great  groups — the  non-metallic  and  the  metallic  ele- 
ments. Belonging  to  the  former  group  we  find  as 
minerals  the  elements  carbon  and  sulphur;  and  in 
the  latter  group  we  have  the  native  metals  copper, 
silver,  gold,  and  platinum,  and  the  so-called  semi- 
metals  arsenic,  antimony,  and  bismuth. 

Carbon,  though  abundant  in  the  organic  world, 
and  present  also  in  chemical  combination  in  lime- 
stone rocks  and  the  mineral  carbonates,  is  of  rare 
occurrence  as  a  native  mineral.  It  is  found  crystal- 
lized in  two  quite  distinct  forms,  which  as  minerals 
are  known  as  diamond  and  graphite.  The  other 
chemical  elements  met  with  as  minerals  are  known 
by  their  ordinary  or  chemical  names,  sometimes  pre- 
53 


54          THE    WORLD'S    MINERALS 

ceded  by  the  word  native:  thus,  we  may  speak  of 
sulphur  or  native  sulphur,  gold  or  native  gold,  etc. 

NON-METALLIC  ELEMENTS 

DIAMOND 

(Plate  i,  Fig.  i). — Diamond  is  perhaps  the  most 
remarkable  and  interesting  of  all  minerals,  and  for 
this  reason  it  will  be  treated  in  rather  more  detail 
than  the  other  species.  One  would  little  imagine  that 
such  a  brilliant,  transparent,  colorless,  and  at  the 
same  time  intensely  hard  gem  is  composed  merely  of 
carbon,  our  usual  idea  of  which  is  a  black,  opaque 
substance,  so  soft  that  it  soils  the  fingers. 

When  heated  in  air,  or  better  still  in  oxygen,  a  dia- 
mond burns  with  a  small  bluish  flame,  and  gradually 
disappears.  The  product  of  combustion  is  the  gas 
carbon  dioxide,  identical  with  that  produced  when 
any  other  form  of  carbon  burns  in  the  air.  Diamond 
is,  in  fact,  pure  carbon ;  but,  conversely,  pure  carbon 
is  not  necessarily  diamond.  We  must  have  the  car- 
bon crystallized,  and,  moreover,  the  crystals  must  be 
of  particular  form,  for,  as  already  mentioned, 
graphite  is  also  a  crystallized  form  of  carbon. 

Crystals  of  diamond  belong  to  the  cubic  system. 
They  most  commonly  present  the  form  of  the  regular 
octahedron — that  is,  a  solid  bounded  by  eight  equi- 
lateral triangles  (Text-Fig,  i,  p.  10).  The  crystal 
seen  partly  embedded  in  the  matrix  in  Plate  i  is  of 
this  form.  Less  frequently  the  crystals  have  the  form 


DIAMOND  55 


of  the  cube  (Text-Fig.  2)  or  the  rhombic-dodeca- 
hedron (Text-Fig.  5).  A  peculiarity  presented  by 
crystals  of  diamond  is  that  their  faces  are  usually 
curved  and  their  edges  rounded,  and  sometimes  this 
rounding  is  so  pronounced  that  the  crystals  are  al- 
most spherical  in  form.  Again,  the  faces  are  fre- 
quently beautifully  marked  with  tiny  triangular  pits 
or  depressions. 

The  great  hardness  of  diamond  is  one  of  its  most 
remarkable  features.  It  is  by  far  the  hardest  of  all 
known  substances,  whether  natural  or  artificial. 
There  is  nothing  that  it  will  not  scratch  with  ease. 
This  was  well  known  to  the  ancients,  who  called 
the  stone  adamas,  on  account  of  its  adamantine  or 
unconquerable  nature;  and  it  is  from  this  word  that 
our  name  diamond  is  derived.  The  ancients  believed 
that  a  diamond  could  not  be  broken,  but  would  rather 
shatter  the  hammer  and  anvil.  There  is,  however, 
an  important  difference  between  hardness  and  fran- 
gibility,  and,  as  a  matter  of  fact,  diamond  is  quite 
brittle. 

Further,  diamond  has  the  peculiar  property  of 
splitting  with  ease  along  certain  directions  in  the 
crystal.  There  are  four  such  directions  of  easy  split- 
ting, or  cleavage,  which  are  parallel  to  the  four  pairs 
of  parallel  faces  of  the  octahedron.  In  these  direc- 
tions the  fractured  surfaces  are  perfectly  plane  and 
smooth. 

The  specific  gravity  of  diamond  is  3.52  (that  is, 
a  diamond  is  three  and  a  half  times  heavier  than  an 
equal  volume  of  water),  being  considerably  higher 


56          THE    WORLD'S    MINERALS 

than  that  of  other  forms  of  carbon ;  the  specific  grav- 
ity of  graphite,  for  example,  is  only  2.2.  We  thus 
see  that  in  diamond  the  ultimate  particles  of  carbon 
must  be  very  closely  packed  together;  and  it  is  this 
close  packing,  according  to  a  regular  and  symmetric- 
al plan,  that  gives  to  diamond  its  particular  crystal- 
line form  and  its  unique  physical  properties. 

Of  other  physical  characters  of  the  diamond,  some 
mention  must  be  made  of  its  optical  properties,  or  its 
behavior  with  respect  to  light,  since  on  these  largely 
depends  its  use  as  a  gem.  The  index  of  refraction 
is  very  high,  and  consequently  a  ray  of  light  traveling 
from  air  into  diamond  will  be  more  strongly  bent,  or 
refracted,  than  a  ray  passing  from  air  into  water  or 
ordinary  glass.  Further,  the  exact  value  of  the  index 
or  refraction  depends  on  the  color  of  the  light,  being 
2.407  for  red  light,  and  2.465  for  violet  light.  A 
prism  of  diamond,  therefore,  gives  a  much  longer 
colored  spectrum  than  many  other  transparent  sub- 
stances; in  other  words,  the  dispersion,  or  dispersive 
power,  is  high.  It  is  owing  to  this  high  dispersive 
power  that  a  faceted  diamond  displays  its  brilliant 
flashes  of  rainbow  colors.  The  high  refractive  index 
is  the  cause  of  the  marked  brilliancy,  or  fire,  shown 
by  a  faceted  diamond.  Connected  also  with  the 
high  refractive  index  we  have  the  characteristic  ada- 
mantine luster,  which  enables  a  diamond  to  be  recog- 
nized at  sight  by  the  experienced  eye.  On  the  rough 
surfaces  of  the  less  clear  stones  this  luster  is  almost 
metallic  in  character,  and,  indeed,  some  crystals  have 
almost  the  appearance  of  metallic  lead. 


DIAMOND  57 


In  the  color  of  diamond  there  is  a  considerable 
range.  Perfectly  pure  crystals  are  colorless  and 
transparent,  or  water-clear,  and  are  described  in  the 
trade  as  stones  of  the  "first  water."  Other  crystals 
range  from  pale  yellow  to  dark  brown  or  even  black, 
while,  less  frequently,  the  color  may  be  pronounced 
shades  of  blue,  green,  or  red.  These  colors  are  due 
to  the  presence  of  mere  traces  of  coloring  matter  in 
the  material  of  the  diamond  itself. 

Diamonds  are  of  quite  local  distribution,  and  in 
Europe  the  only  recorded  occurrence  is  in  the  Ural 
Mountains.  The  principal  diamond-producing  coun- 
tries are  South  Africa,  India,  and  Brazil;  other  oc- 
currences of  less  importance  are  those  of.  Borneo, 
New  South  Wales,  British  Guiana,  and  the  United 
States  of  North  America.  Although  the  gem  has 
been  obtained  from  India  ever  since  the  time  of  the 
Romans,  it  was  not  discovered  in  Brazil  until  the 
year  1725,  and  in  South  Africa  not  until  1867.  At 
the  present  time  the  diamond-mining  industry  of 
South  Africa  is  in  a  very  flourishing  condition,  and 
far  more  diamonds  have  been  found  there  than  in 
India  and  Brazil  together  for  centuries. 

The  usual  mode  of  occurrence  is  in  the  sands  and 
gravels  of  river-beds.  By  a  simple  process  of  wash- 
ing in  a  shallow  dish,  called  in  Brazil  a  "batea,"  the 
lighter  materials  are  carried  away  in  a  stream  of 
water,  leaving  behind  the  diamond  with  the  other 
denser  stones.  In  India  and  Brazil  some  of  these 
diamantiferous  deposits  are  of  great  antiquity,  and 
the  sand  and  pebbles  are  cemented  together  to  form 


58          THE   WORLD'S   MINERALS 

a  solid  rock  known  as  conglomerate.  The  materials 
of  these  secondary  deposits  must  clearly  have  been 
derived  by  the  weathering  and  breaking  down  of 
pre-existing  rocks;  but  in  India  and  Brazil  the 
original  rock  which  supplied  the  diamond  has  never 
yet  been  discovered. 

In  South  Africa,  on  the  other  hand,  the  diamonds, 
which  were  first  found  amongst  the  gravels  of  the 
Vaal  River,  were  very  soon  traced  to  volcanic  pipes 
in  the  neighborhood;  and  when  this  discovery  was 
made  the  town  of  Kimberley  very  quickly  sprang  up 
on  the  spot. 

These  volcanic  pipes  are  of  a  rather  special  kind. 
At  the  surface  they  have  a  more  or  less  circular  or 
oval  outline,  with  a  diameter  of  200  to  300  yards,  and 
they  project  only  slightly  above  the  general  level  of 
the  ground.  Downwards  they  extend  to  unknown 
depths,  passing  through  horizontal  strata  of  shale, 
diabase,  and  quartzite.  The  material  filling  the 
pipes  is  known  locally  as  "blue  ground,"  though 
usually  it  is  more  of  a  dark  greenish  shade,  as  rep- 
resented in  Plate  i.  It  is  really  a  mixture  of  ma- 
terials, and  consists  largely  of  the  alteration  products 
of  rocks  rich  in  the  mineral  olivine.  Chemically, 
the  blue  ground  is  essentially  a  hydrated  silicate  of 
magnesium.  In  structure  the  material  is  broken  and 
fragmentary,  and  it  has  no  doubt  been  brought  up 
into  the  pipes  from  an  underground  reservoir  of 
molten  rock  by  a  series  of  steam  explosions  under 
high  pressure.  There  does  not  appear  to  have  been 
any  active  volcano  discharging  material  at  the  earth's 


DIAMOND  59 


surface.  The  diamonds  are  found  embedded  in  this 
material;  that  they  also  were  brought  up  into  the 
pipes  by  the  same  series  of  explosions  is  abundantly 
proved  by  the  fact  that  the  crystals  are  often  broken 
and  fragmentary.  The  blue  ground  is  thus  not  the 
original  mother-rock  of  the  diamond,  and  the  exact 
nature  of  this  still  remains  unknown,  though  prob- 
ably it  was  an  olivine-rock. 

The  actual  amount  of  diamond  present  in  the  blue 
ground  is  relatively  very  small,  only  2  to  5  millionths 
per  cent.,  and  a  specimen  showing  a  crystal  actually 
embedded  in  the  matrix  (as  in  Plate  i,  representing 
a  specimen  from  Kimberley)  is  a  rarity.  Large 
quantities  of  the  rock  have  therefore  to  be  extracted 
by  mining  operations — from  the  upper  portions  of 
pipes  by  open  workings,  and  at  greater  depths  by  a 
regular  system  of  underground  mining.  The  ex- 
cavated rock  is  spread  out  on  extensive  and  specially 
guarded  "floors,"  where  it  is  watered,  harrowed,  and 
exposed  to  the  weather  for  about  a  year.  After  this 
treatment  it  is  soft  enough  for  crushing,  and  the 
heavy  minerals  (diamond,  garnet,  ilmenite,  etc.)  are 
separated  by  specially  constructed  washing  machin- 
ery. From  this  heavy  residue  the  diamonds  were 
formerly  picked  out  by  hand,  but  now  it  is  passed 
over  by  a  vibrating  table  with  a  greased  surface;  the 
diamonds  have  the  curious  property  of  adhering  to 
the  grease,  while  the  other  minerals  slide  off. 

In  the  neighborhood  of  Kimberley,  in  Cape  Col- 
ony, there  are  several  of  these  diamond-bearing  pipes, 
and  there  are  others  in  Orange  River  Colony  and  in 


60          THE   WORLD'S   MINERALS 

the  Transvaal.  By  far  the  largest  pipe  is  one  situated 
about  twenty  miles  W.N.W.  of  Pretoria,  which  was 
discovered  in  1902,  and  is  worked  as  the  Premier 
mine.  This  pipe  measures  half  a  mile  across,  and  it 
is  further  remarkable  in  having  produced  the  largest 
known  diamond,  namely,  the  "Cullinan." 

Another  mode  of  occurrence  of  diamond  remains 
to  be  mentioned — one  which,  though  of  no  practical 
importance,  is  of  considerable  scientific  interest.  Mi- 
croscopic diamonds  have  been  detected  in  a  few  of 
the  meteoric  stones  and  irons  which  as  shooting  stars 
have  fallen  to  our  earth  from  surrounding  space. 

This  mode  of  occurrence  has  a  bearing  on  the 
artificial  production  of  diamond.  By  dissolving  car- 
bon in  molten  iron  and  by  cooling  the  mass  rapidly 
under  an  enormous  pressure,  microscopic  diamonds 
have  resulted;  and  similar  results  have  been  ob- 
tained by  dissolving  carbon  in  molten  olivine. 

As  to  the  practical  applications  of  diamond,  its 
use  in  jewelry  is  too  well  known  to  need  more  than 
a  passing  mention.  In  the  operation  of  fashioning 
a  faceted  gem,  the  rough  stone  is  first  reduced  to  a 
suitable  size  and  shape  by  taking  advantage  of  the 
property  of  cleavage.  The  general  form  is  then  pro- 
duced by  rubbing  two  diamonds  together — a  process 
known  as  bruting;  and  the  facets  are  finished  by 
grinding  on  a  rapidly  revolving  disc  or  lap  of  iron, 
the  grinding  material  being  diamond  powder  itself. 
The  form  of  cutting  most  usually  adopted  is  the  bril- 
liant, and  cut  diamonds  are  consequently  often  known 
in  the  trade  simply  as  brilliants.  For  small,  flat 


DIAMOND  61 


stones  the  less  effective  rose-cut  is  the  form  sometimes 
employed. 

Certain  important  technical  applications  of  the 
diamond  depend  on  the  great  hardness  of  the  mater- 
ial, which,  as  already  mentioned,  far  exceeds  that  of 
all  other  substances.  The  glazier's  diamond  consists 
of  a  crystal  with  a  naturally  rounded  edge;  sharp 
cleavage  splinters  of  diamond  are  used  for  writing 
on  glass.  Diamond  powder  is  much  used  by  lapi- 
daries and  gem-cutters,  it  being,  in  fact,  the  only 
material  with  which  diamond  can  be  ground  and 
polished.  Extensive  use  is  made  of  diamond  for 
rock-drills;  the  cloudy,  rounded  crystals  known  as 
bortf  and  the  black,  granular  variety  called  carbon- 
ado, being  used  for  this  purpose.  The  stones  are 
embedded  by  pressure  in  the  steel  crown  of  the  drill. 

We  cannot  conclude  this  account  of  the  mineral 
diamond  without  reference  to  some  of  the  more  note- 
worthy of  the  large  and  famous  cut  gems,  about 
which,  indeed,  it  would  be  possible  to  write  a  whole 
volume  of  romance. 

Several  large  gems  were  seen  in  India  by  the 
French  traveler  Tavernier  about  the  middle  of  the 
seventeenth  century.  Large  Indian  diamonds  were, 
however,  known  in  Europe  long  before  that  time,  for 
Charles  the  Bold,  Duke  of  Burgundy  (1433-77), 
was  the  possessor  of  the  "Florentine"  and  the  "Sancy" 
diamonds.  After  many  vicissitudes  of  fortune  these 
gems  are  now,  one  with  the  Austrian  crown  jewels, 
and  the  other  in  the  possession  of  an  Indian  prince; 
they  weigh  133^  and  53  *A  carats,  respectively. 


62          THE   WORLD'S    MINERALS 

The  "Koh-i-noor"  (a  name  meaning  "Mountain 
of  Light")  was  taken  in  1739  by  Nadir  Shah,  the 
Persian  conqueror  of  the  Mogul  Empire,  and  in 
1850  it  was  presented  by  the  East  India  Company  to 
Queen  Victoria.  In  its  Indian  form  of  cutting  it 
weighed  186^  carats,  but  this  was  reduced  by  re- 
cutting  in  England  to  106^  carats. 

Most  of  the  historic  diamonds  are  of  Indian  origin, 
only  one  or  two  coming  from  Brazil.  But  during 
recent  years  much  larger  stones  have  been  found  in 
comparatively  large  numbers  in  the  South  African 
diamond-mines.  These  are,  indeed,  sometimes  too 
large  for  cutting  into  a  single  gem,  and  they  have  to 
be  divided  by  cleavage.  This  was  the  case  with  the 
"Excelsior,"  a  stone  of  969^  carats,  found  in  1893 
in  the  Jagersfontein  mine,  Orange  River  Colony; 
and  also  the  "Cullinan,"  which  was  found  in  1905 
in  the  Premier  mine  near  Pretoria,  Transvaal.  The 
"Cullinan"  is  by  far  the  largest  diamond  yet  dis- 
covered, weighing  in  the  rough  no  less  than  3025  H 
English  carats  (=621.2  grams),  or  nearly  22  oz. 
avoirdupois.  It  was  presented  by  the  Transvaal  gov- 
ernment to  King  Edward  VII.  in  1907,  and  has  since 
been  cut  into  nine  large  brilliants  and  ninety-six 
small  brilliants.  The  two  largest  brilliants,  weigh- 
ing 516^  and  309^  carats,  are  each  far  larger  than 
any  other  cut  diamond. 


NON-METALLIC  ELEMENTS. 


Plate  1 


1,  Diamond.    2,  Graphite.    3,  Sulphur., 


GRAPHITE  63 


GRAPHITE 

(Plate  i,  Fig.  2). — Like  diamond,  this  consists 
simply  of  the  chemical  element  carbon ;  but  although 
these  two  minerals  are  identical  in  chemical  com- 
position, they  exhibit  the  most  marked  differences 
in  their  physical  characters.  Diamond  is  the  hardest, 
while  graphite  is  the  softest  of  minerals.  Diamond 
is  colorless  and  perfectly  transparent,  while  graphite 
is  black  and  opaque;  diamond  is  a  non-conductor  of 
electricity,  and  graphite  is  a  good  conductor.  There 
is  also  a  very  marked  difference  in  the  specific  grav- 
ity, that  of  diamond  being  3.52,  while  that  of 
graphite  is  only  2.2.  Again,  while  diamond  is  nearly 
always  found  as  well-developed  octahedral  crystals, 
graphite  is  only  rarely  found  as  small  and  indistinct, 
six-sided  scales.  More  usually  the  material  is  very 
imperfectly  crystallized,  taking  the  form  of  scaly, 
lamellar,  or  fibrous  masses.  These  striking  differ- 
ences between  diamond  and  graphite  are  to  be  at- 
tributed to  the  different,  but  in  each  case  orderly, 
grouping  of  the  ultimate  particles  of  carbon — that 
is,  to  a  difference  in  the  crystalline  structure  of  the 
material. 

Scales  of  graphite  may  be  readily  divided  into  still 
thinner  leaves;  in  other  words,  they  possess  a  perfect 
cleavage  parallel  to  their  large  surface.  The  ma- 
terial is  iron-black  in  color,  with  a  metallic  luster. 
It  is  greasy  to  the  touch,  and  soils  everything  it  comes 
in  contact  with.  On  this  depends,  6f  course,  its  well- 


64          THE    WORLD'S    MINERALS 

known  use  for  making  pencils  (the  so-called  "lead, 
pencils,"  which  contain  no  lead!) ;  and,  indeed,  the 
name  graphite  is  derived  from  the  Greek  word 
meaning  "to  write."  Plumbago  is  another  name  for 
this  mineral. 

The  better  qualities  of  material,  such  as  are  used 
for  making  pencils,  contain  a  certain  amount  of  im- 
purity, which  remains  behind  as  ash  when  the  graph- 
ite is  burnt.  Inferior  qualities,  containing  still  more 
impurity,  are  used  for  making  stove-polish,  foundry 
facings,  crucibles,  etc.  The  purified  material,  con- 
sisting of  fine  scales  floated  off  in  water,  is  employed 
as  a  dry  lubricant. 

Graphite  occurs  mainly  in  the  older  crystalline 
rocks,  in  which  it  forms  veins  or  irregular  masses; 
and  it  is  mined  principally  in  Ceylon,  Siberia,  the 
State  of  New  York,  and  Canada.  The  famous  Bor- 
rowdale  mine,  near  Keswick,  in  Cumberland,  was 
worked  since  the  beginning  of  the  seventeenth  cen- 
tury, and  possibly  earlier;  but  this  mine  is  now 
exhausted.  At  the  present  time  a  certain  amount  of 
commercial  graphite  is  prepared  artificially  in  the 
electric  furnace. 

SULPHUR 

(Plate  i,  Fig.  3). — This  mineral  frequently  occurs 
beautifully  crystallized,  and  the  crystals  are  trans- 
parent and  of  a  bright  primrose-yellow  color.  Pretty 
specimens  like  that  represented  in  Plate  i  are  not  at 
all  uncommon,  and  with  their  bright  and  attractive 
color  they  form  conspicuous  objects  in  any  collection 


SULPHUR  65 


of  minerals.  In  this  picture  we  see  several  crystals 
with  sharply  defined  planes  and  edges  scattered  over 
the  surface  of  a  matrix  consisting  of  minutely  crys- 
tallized calcite.  The  characteristic  form  of  crystals 
is  clearly  shown;  they  are  rhombic  (four-faced) 
pyramids,  truncated  by  a  rhomb-shaped  face  at  their 
summits.  The  system  of  crystallization  is  ortho- 
rhombic,  and  the  crystals  have  three  planes  of  sym- 
metry. 

The  crystal  faces  are  always  very  bright  and 
smooth,  with  a  luster  like  that  of  resin.  There  is  no 
cleavage,  and  the  fractured  surfaces  are  typically 
conchoidal.  The  material  is  quite  soft  (H.  —  2), 
though  very  brittle;  and  it  is  not  at  all  heavy  (sp. 
gr.  =  2.07).  It  is  a  bad  conductor  of  heat;  and  for 
this  reason  a  crystal  of  sulphur  when  held  in  the 
warm  hand  emits  a  crackling  noise  and  becomes 
fissured.  Good  specimens  must,  therefore,  be  han- 
dled with  care. 

The  chemical  properties  of  sulphur  are  well 
known  to  students  of  chemistry.  When  heated,  the 
material  readily  melts  to  a  dark-brown,  viscous 
liquid,  and  in  the  air  it  burns  with  a  small  blue 
flame,  producing  a  most  penetrating  and  suffocating 
odor.  The  product  of  combustion  is  the  gas  sulphur 
dioxide  (SO2),  the  sulphur  having  entered  into 
chemical  combination  with  the  oxygen  of  the  air. 
Sulphur  is  dissolved  by  the  liquid  carbon  disulphide, 
and  as  the  solution  slowly  evaporates  it  deposits  bril- 
liant rhombic  pyramids  of  the  same  form  as  the 
natural  crystals. 


66          THE    WORLD'S    MINERALS 

Like  the  element  carbon,  sulphur  also  exists  in 
more  than  one  crystalline  modification.  When  mol- 
ten sulphur  solidifies,  it  takes  the  form  of  long,  spear- 
shaped  crystals,  which  belong  to  the  monoclinic  sys- 
tem, and  have  the  lower  specific  gravity  of  1.86. 
These  crystals  are,  however,  not  stable  at  the  ordinary 
temperature.  After  a  short  time  cloudy  patches  of 
a  pale  yellow  color  make  their  appearance  in  the 
amber-yellow  crystals;  these  gradually  spread,  and 
finally  the  crystals  fall  to  powder.  Under  the  micro- 
scope this  powder  is  seen  to  consist  of  minute  ortho- 
rhombic  crystals.  There  has  thus  been  a  spontaneous 
conversion  of  the  unstable  monoclinic  sulphur  to  the 
more  stable  orthorhombic  sulphur;  and  for  this  rea- 
son the  monoclinic  modification  of  sulphur  is  un- 
known as  a  natural  mineral.  On  the  other  hand, 
the  two  crystalline  modifications  of  carbon  are  both 
stable  at  the  ordinary  temperature,  and  they  are  con- 
sequently both  found  in  nature  as  minerals. 

Sulphur  is  found  in  all  volcanic  districts,  it  being 
a  common  product  of  sublimation  from  the  sulphur- 
ous gases  of  volcanoes.  This  sublimed  sulphur  is, 
however,  never  distinctly  crystallized;  it  forms 
powdery  and  earthy,  or  sometimes  more  compact  and 
stalactitic,  masses.  The  beautifully  crystallized  speci- 
mens of  sulphur  are  found  in  crevices  in  beds  of  clay 
and  marl,  together  with  crystals  of  gypsum,  calcite, 
aragonite,  and  celestite. 

The  best-known  locality  for  such  crystals  is  in  the 
neighborhood  of  Girgenti,  in  Sicily — a  district  far 
removed  from  Mount  Etna — and  the  materials  here 


SULPHUR  67 


are  not  of  volcanic  origin.  The  specimen  represented 
in  Plate  I  is  from  this  locality.  In  this  district  there 
are  numerous  sulphur-mines.  The  native  sulphur  is 
separated  from  its  matrix  by  fusion,  and  the  crude 
sulphur  so  obtained  is  further  purified  by  sublima- 
tion, giving  "flowers  of  sulphur."  This  is  then  fused 
and  cast  into  sticks  to  give  the  roll-sulphur  of 
commerce. 

Sulphur  has  many  important  technical  applica- 
tions. It  is  used  in  medicine;  in  the  manufacture  of 
matches,  gun-powder,  and  fireworks;  for  vulcanizing 
india-rubber;  in  the  preparation  of  sulphuric  acid 
(oil  of  vitriol) ,  ultramarine,  and  many  other  sulphur 
compounds.  Much  of  the  oil  of  vitriol  of  commerce 
is  now  prepared  from  iron-pyrites,  but  this  is  not  so 
pure  as  that  obtained  from  sulphur.  The  sulphur 
dioxide  produced  by  burning  sulphur  in  the  air  is 
extensively  used  for  disinfecting  and  bleaching  pur- 
poses, and  in  the  boiling  of  wood-pulp  employed  in 
the  manufacture  of  paper. 

In  chemical  combination  sulphur  is  present  in  sev- 
eral kinds  of  minerals.  The  sulphides  are  compounds 
of  sulphur  with  a  metal,  and  the  sulphates  contain 
oxygen  in  addition  to  sulphur  and  a  metal. 

SEMI-METALLIC  ELEMENTS 

In  this  class  we  have  the  native  elements  arsenic, 
antimony,  and  bismuth,  which,  though  possessed  of 
a  metallic  luster  and  high  specific  gravity,  approach 
the  non-metals  in  some  of  their  chemical  properties. 


68          THE    WORLD'S    MINERALS 

They  are  closely  related  to  each  other  chemically, 
being  isomorphous,  and  form  a  progressive  series 
with  arsenic  at  the  non-metallic  and  bismuth  at  the 
metallic  end  of  the  series.  The  progressive  nature 
of  this  series  is  well  shown  by  the  specific  gravities, 
which  are  5.7,  6.7,  and  9.8,  respectively,  in  arsenic, 
antimony,  and  bismuth.  As  minerals  they  are  of 
comparatively  rare  occurrence,  and  for  commercial 
purposes  these  elements  are  more  often  extracted 
from  their  compounds,  which  occur  in  nature  mostly 
as  sulphides. 

ARSENIC 

(Plate  2,  Fig.  i). — Native  arsenic  is  usually  found 
as  masses  with  a  rounded  (mamillated)  surface  and 
an  internal  shelly  structure.  The  characteristic  form 
of  surface  is  represented  in  the  picture.  Freshly 
fractured  surfaces  are  tin-white,  with  a  metallic  lus- 
ter; but  on  exposure  to  the  air  these  very  soon  tarnish, 
and  become  dull  and  dark  grey. 

This  mineral  occurs  in  metalliferous  veins,  to- 
gether with  ores  of  silver  and  cobalt.  It  is  found  in 
the  Harz  Mountains  and  in  Saxony.  Globular 
masses  of  radiating  crystals  are  found  embedded  in 
clay  at  Akatani,  in  Japan. 

ANTIMONY 

(Plate  2,  Fig.  2). — Native  antimony  is  found  as 
granular  or  lamellar  masses,  which  have  a  tin-white 
color  and  metallic  luster.  There  is  a  perfect  cleav- 


BISMUTH  69 


age  in  one  direction.  It  occurs  in  metalliferous  veins, 
usually  with  ores  of  silver,  at  but  few  localities — for 
example,  in  Sweden  and  Borneo. 

BISMUTH 

(Plate  2,  Fig.  3). — Native  bismuth  is  met  with  as 
reticulated  or  feathery  forms,  and  also  as  granular 
masses.  It  usually  shows  a  yellowish  tarnish,  but  a 
freshly  fractured  surface  is  silver-white,  with  a  char- 
acteristic tinge  of  red.  It  is  found  in  metalliferous 
veins  in  Cornwall,  Saxony,  Bolivia  and  Queensland. 

Distinctly  developed  crystals  of  bismuth,  though 
rare  in  nature,  are  readily  prepared  artificially  by 
fusion.  The  metal  is  melted  in  a  crucible,  and  after 
it  has  cooled  for  a  little  while  the  crust  is  broken  and 
the  still  molten  portion  poured  out.  The  interior  of 
the  crucible  is  then  seen  to  be  completely  lined  with 
sharply  formed  crystals,  which  have  sunken  faces 
and  a  brilliantly  colored  iridescent  tarnish  on  their 
surface.  Very  pretty  groups  of  crystals  are  obtained 
in  this  way.  These  crystals  have  the  appearance  of 
cubes,  but  in  reality  are  rhombohedra,  the  faces  be- 
ing not  quite  at  right  angles  to  one  another.  Fur- 
ther, there  being  only  one  direction  of  cleavage  (per- 
pendicular to  the  principal  axis),  only  two  of  the 
opposite  corners  of  the  apparent  cubes  can  be  trun- 
cated by  cleavage. 

A  certain  quantity  of  commercial  bismuth  is  ob- 
tained from  native  bismuth,  especially  in  Bolivia, 
and  the  remainder  is  extracted  from  the  sulphide 


70          THE    WORLD'S    MINERALS 

(bismuthinite,  Bi2S8).  It  is  largely  used  for  making 
readily  fusible  alloys,  which  find  an  application  in 
the  safety-plugs  of  boilers  and  fire-extinguishers. 
Salts  of  bismuth  are  extensively  used  in  medicine  for 
relieving  indigestion. 

METALLIC  ELEMENTS 

SILVER 

(Plate  2,  Figs.  4  and  5). — The  useful  metal  silver 
occurs  native  as  a  mineral,  but  as  a  source  of  the 
metal  the  various  ores  of  silver  are  of  much  greater 
importance,  while  a  considerable  amount  is  extracted 
from  ores  of  lead,  which  always  contain  a  small  pro- 
portion of  silver. 

Native  silver  is  found  principally  in  the  upper 
levels  of  silver-mines,  it  having  resulted  by  the  sec- 
ondary alteration,  or  weathering,  of  various  silver- 
bearing  minerals.  It  is  met  with  in  cavities  in  the 
ore  as  fine,  twisted,  wire-like  forms.  This  form  is 
extremely  characteristic  of  native  silver;  the  wires 
are  usually  finer  than  represented  in  the  picture 
(Fig.  5).  Under  these  conditions  it  is  found  in 
Cornwall  and  Saxony,  and  more  abundantly  in  the 
silver-mines  of  Mexico  and  Chile. 

Crystals  of  native  silver  are  quite  rare.  They  be- 
long to  the  cubic  system,  and  usually  have  the  form 
of  distorted  cubes  (Fig.  4).  Most  of  the  crystallized 
specimens  to  be  seen  in  mineral  collections  have  come 
from  the  silver-mines  at  Konigsberg,  in  Norway,  this 


METALLIC  ELEMENTS. 


Plate  2. 


4 


1,  Arsenic.    2,  Antimony.    3,  Bismuth.     4,  ^  Silver: 


SILVER— PLATINUM  71 

being  the  locality  of  the  specimen  represented  in 
Fig.  4.  Masses  of  native  silver  weighing  as  much 
as  5  cwt.  have  been  found  in  these  mines. 

The  specific  gravity  of  native  silver  ranges  from 
10  to  n,  this  variation  being  due  to  the  presence 
of  small  amounts  of  gold  or  copper  alloyed  with 
the  silver. 

PLATINUM 

(Plate  3,  Fig.  i). — This  metal  was  first  brought 
to  Europe  in  the  year  1735  from  Colombia,  South 
America.  Being  a  white  metal,  resembling  silver  in 
appearance,  it  was  called  platina  in  Spanish,  plata 
being  the  Spanish  name  for  silver.  It  differs  from 
silver,  however,  in  its  very  high  specific  gravity, 
which  ranges  from  14  to  19  in  the  native  metal  (this 
variation  being  due  to  the  presence  of  impurities, 
principally  iron),  and  is  as  high  as  21.5  in  the  chem- 
ically pure  metal.  With  the  exception  of  iridium 
and  osmium,  it  is  the  heaviest  of  metals,  and,  indeed, 
of  all  kinds  of  matter.  It  also  differs  from  silver  in 
being  fusible  only  with  great  difficulty,  and  in  being 
very  resistant  to  acids. 

These  latter  properties  make  the  metal  invaluable 
for  the  construction  of  certain  kinds  of  chemical  ap- 
paratus, such  as  crucibles,  basins,  wire,  and  foil.  The 
metal  is  of  rare  occurrence,  and  owing  to  the  limited 
supply  the  price  is  constantly  rising.  Formerly  the 
metal  was  coined  in  Russia,  but  now  its  value  is 
greater  than  that  of  gold. 

Native  platinum  is  found  as  grains  and  small  nug- 


72          THE    WORLD'S    MINERALS 

gets  (Plate  3,  Fig.  i)  in  the  beds  of  streams.  Par- 
ticles of  chromite  (chrome  iron-ore)  are  sometimes 
seen  attached  to  the  nuggets,  proving  that  the  ma- 
terial has  been  derived  from  the  olivine-rocks  which 
outcrop  in  the  neighborhood.  Practically  the  whole 
of  the  platinum  used  commercially  comes  from  the 
Ural  Mountains. 

The  only  source  of  platinum  is  the  native  metal, 
but  the  element  is  also  known  to  occur  in  the  tiny 
crystals  of  the  rare  mineral  sperrylite,  an  arsenide  of 
platinum,  found  in  Canada. 

GOLD 

(Plate  3,  Figs.  2-4). — Though  of  sparing  occur- 
rence, gold  is  a  very  widely  distributed  mineral,  and 
it  is  found  in  the  native  state  in  almost  every  country 
of  the  world.  Being  found  as  grains  and  nuggets  in 
the  beds  of  streams  and  rivers,  it  must  have  attracted 
the  attention  of  man  at  a  very  early  period,  and  gold 
ornaments  have  been  unearthed  from  the  burial- 
places  of  prehistoric  man. 

Well-shaped  crystals  of  gold  are  small  and  very 
rare;  they  have  the  form  of  the  regular  octahedron 
or  cube,  usually  with  rounded  edges.  A  small  group 
of  gold  crystals  is  represented  in  Plate  3,  Fig.  2. 
Thin  plates  of  gold  (so-called  leaf-gold),  bearing 
delicate  crystalline  markings  on  their  surfaces,  are 
rather  more  frequent,  and  examples  from  Transyl- 
vania in  Hungary,  and  Colombia  in  South  America, 
are  well  known.  Alluvial  gold,  as  found  in  the  beds 


GOLD  73 


of  streams  and  rivers,  has  the  form  of  scales,  grains, 
and  rounded  nuggets;  while  in  gold-quartz,  the 
precious  metal  takes  the  form  of  veins  and  irregular 
patches  (Figs.  3  and  4).  The  largest  mass  of  native 
gold  on  record  was  a  nugget  called  the  "Welcome 
Stranger,"  found  in  Victoria,  Australia,  in  the  year 
1869;  it  weighed  156  Ib.  avoirdupois,  and  was  val- 
ued at  £9534. 

Native  gold  is  never  quite  pure,  but  is  always 
alloyed  with  from  i  to  38  per  cent,  of  silver.  The 
greater  the  amount  of  silver  present,  the  lower  is  the 
specific  gravity  (which  ranges  from  15  to  19),  the 
paler  the  yellow  color,  and  the  greater  the  hardness. 

Pure  gold  is  as  soft  as  lead — too  soft,  indeed,  for 
use  in  coinage  and  jewelry.  Its  hardness  and  dur- 
ability are  increased  by  alloying  it  with  copper,  or 
sometimes  with  silver.  The  amount  of  gold  actually 
present  in  such  an  alloy  is  expressed  as  so  many  carats 
— that  is,  as  so  many  parts  in  24.  Thus,  22-carat  gold 
(as  in  the  English  gold  coinage)  means  that  in  each 
24  parts  of  metal  22  parts  are  of  gold  and  2  of  base 
metal;  and  in  i2-carat  gold  only  half  is  pure  gold. 

Gold  is  separated  from  the  sand  and  gravel  of 
alluvial  deposits  by  a  simple  process  of  washing, 
whereby  the  lighter  materials  are  carried  away  in  a 
stream  of  water,  leaving  the  heavier  gold  behind. 
The  finer  particles  are  collected  from  this  heavy 
residue  by  the  addition  of  mercury,  with  which  gold 
readily  forms  an  amalgam.  On  heating  the  amal- 
gam the  mercury  is  volatilized,  leaving  the  gold. 
Being  easily  worked,  such  incoherent  deposits  are  al- 


74          THE   WORLD'S    MINERALS 

ways  the  first  to  be  attacked,  and  in  many  countries 
they  are  now  exhausted.  Recourse  must  then  be  had 
to  the  solid  rocks,  the  working  of  which  is  a  far 
more  laborious  operation.  The  gold-bearing  rock 
taken  from  the  mine  is  crushed  to  powder  under 
heavy  stamps,  and  the  gold  is  dissolved  by  a  dilute 
solution  of  potassium  cyanide — a  process  known  as 
the  cyanide  process.  The  gold  is  deposited  from 
this  solution  by  means  of  an  electric  current. 

The  mother-rock,  or  matrix,  of  gold  is  very  often 
a  white  quartz  (Plate  3,  Figs.  3  and  4) ,  which  occurs 
as  veins,  traversing  the  older  crystalline  rocks.  In 
the  rich  goldfields  of  the  Witwatersrand,  or  the 
"Rand,"  in  the  Transvaal,  the  matrix  is,  however,  a 
hard,  compact  conglomerate,  known  locally  as  "ban- 
ket," consisting  of  pebbles  of  quartz  cemented  to- 
gether by  a  siliceous  material.  In  this  rock  the  gold 
is  so  finely  divided  that  only  rarely  is  it  visible  to  the 
unaided  eye.  Gold  is  also  found  in  metalliferous 
veins,  together  with  iron-pyrites,  ores  of  silver,  etc. 
In  some  of  these  veins  the  gold  exists  not  only  in  the 
native  state,  but  also  in  chemical  combination  with 
tellurium.  These  tellurides  of  gold  are  of  consider- 
able importance  as  a  source  of  gold  at  Kalgoorlie, 
in  Western  Australia,  and  at  Cripple  Creek,  in 
Colorado. 

When  these  solid  gold-bearing  rocks  are  broken 
down  by  the  slow  action  of  weathering  agencies  and 
their  materials  transported  by  running  water,  many 
of  the  accompanying  minerals  are  destroyed;  but  the 
quartz  and  the  gold,  being  indestructible,  accumu- 


METALLIC  ELEMENTS. 


Plate  3. 


1,  Platinum.     2—4,  Gold.  5,  6,  Coppef.r,:  ;  V;      \  ;  ; 


GOLD— COPPER  75 

late  in  the  beds  of  streams  and  rivers,  forming  the 
alluvial  deposits  already  mentioned.  In  this  way  the 
work  of  extracting  gold  from  the  solid  rock  is  per- 
formed by  nature  herself. 

In  the  British  Isles,  small  nuggets,  the  largest 
weighing  27  oz.,  have  been  found  in  Cornwall,  Scot- 
land, and  County  Wicklow.  During  the  sixteenth 
century  there  were  quite  extensive  gold-washings  in 
the  Leadhills  district  in  Lanarkshire;  while  at  the 
present  day  a  thousand  or  more  ounces  of  gold  are 
annually  extracted  from  the  quartz-veins  in  the 
neighborhood  of  Dolgelly,  in  Merionethshire. 

The  principal  gold-producing  countries  are  the 
Transvaal,  Australia,  California  and  Alaska.  In  the 
year  1907  the  production  for  the  whole  world 
amounted  to  604  tons  of  fine  gold,  and  of  this  amount 
365  tons  was  produced  in  the  British  Empire. 

COPPER 

(Plate  3,  Figs.  5  and  6). — Like  gold  and  silver, 
native  copper  crystallizes  in  the  cubic  system,  but 
distinctly  formed  crystals  are  here  also  of  rare  oc- 
currence. Well-shaped  cubes,  such  as  represented  in 
Fig.  5,  are  quite  exceptional,  and  even  the  complex 
twinned  crystals  shown  in  Fig.  6  are  but  rarely  met 
with.  Usually  the  metal  has  the  form  of  thin  plates, 
filling  narrow  crevices  in  rocks  of  various  kinds,  such 
as  sandstone,  slate,  or  igneous  rocks;  and  these  plates 
frequently  have  a  delicate  frond-like  or  dendritic 
structure.  Mossy  aggregates  are  also  common,  par- 


76          THE   WORLD'S   MINERALS 

ticularly  in  the  upper  portions  of  veins  of  copper- 
ore,  where  the  material  has  resulted  by  the  decom- 
position of  various  copper-bearing  minerals. 

On  its  surface,  native  copper  is  usually  dull  and 
tarnished,  with  a  dark  chocolate-brown  color,  or 
sometimes  green  (owing  to  the  surface  alteration  of 
the  material  to  green  carbonate  of  copper) ;  and  only 
on  a  fresh  fracture  is  the  characteristic  copper-red 
color,  with  bright  metallic  luster,  to  be  seen.  The 
native  metal  is  usually  almost  chemically  pure  cop- 
per, and  its  specific  gravity  does  not  vary  much 
from  8.9. 

Small  specimens  of  native  copper  are  found  in 
most  copper-mines;  but  as  a  source  of  metal  this 
mineral  is  of  far  less  importance  than  copper-pyrites 
and  other  compounds  of  copper.  In  the  State  of 
Michigan,  on  the  south  shore  of  Lake  Superior,  it 
exists  in  large  quantities  and  is  extensively  mined; 
but  owing  to  the  toughness  of  the  metal,  the  mining 
operations  are  attended  with  some  difficulty.  One 
mass  of  pure  copper  found  at  this  locality  was  esti- 
mated to  weigh  as  much  as  420  tons.  In  the  old 
copper-mines  in  the  Lizard  district  of  Cornwall, 
masses  of  copper  up  to  3  tons  in  weight  were  found 
embedded  in  the  serpentine  rock. 

IRON 

We  cannot  conclude  our  description  of  the  native 
elements  without  adding  a  brief  mention  of  native 
iron.  Although  iron,  in  the  form  of  its  various 


IRON  77 


chemical  compounds,  is  of  such  abundance  amongst 
the  materials  of  the  earth's  crust,  yet  native  iron  is 
of  the  greatest  rarity.  This  is  readily  explained  by 
the  fact  that  metallic  iron  is  readily  altered  by  oxida- 
tion when  exposed  to  the  weather.  Small  grains 
of  metallic  iron  have  been  observed  embedded  in 
basaltic  rocks  at  a  few  places,  and  in  Greenland  large 
masses  have  been  found  in  basalt.  But  of  special 
interest  are  the  masses  of  metallic  iron  (invariably, 
however,  alloyed  with  about  9  per  cent,  of  nickel) 
which  occasionally  fall  to  our  earth  from  outside 
space.  Such  a  mass  of  meteoric  iron,  weighing  j%. 
lb.,  fell  at  Rowton,  in  Shropshire,  on  April  20,  1876; 
and  a  mass  weighing  as  much  as  50  tons  has  been 
found  in  Mexico. 


CHAPTER  VI 

THE  SULPHIDES,  ARSENIDES,  AND 
SULPHUR-SALTS 

IN  this  class  we  have  the  sulphides  and  arsenides 
of  the  heavy  metals,  and  also  a  few  rarer  minerals, 
in  which  both  sulphur  and  arsenic  (or  antimony) 
enter  into  chemical  combination  with  a  heavy  metal. 
Most  of  the  minerals  to  be  here  described  are  of 
importance  to  the  miner  as  ores.  They  are  usually 
heavy  and  opaque,  with  a  metallic  luster,  thus  some- 
what resembling  metals  in  their  appearance;  but 
some  are  quite  transparent  and  brightly  colored. 
They  occur,  for  the  most  part,  in  mineral-veins,  or 
lodes,  which  intersect  the  older  rocks  of  the  earth's 
crust.  The  different  kinds  are  frequently  found  to- 
gether in  the  same  vein,  and  they  are  also  much  inter- 
mixed with  various  sparry  minerals.  It  is,  there- 
fore, part  of  the  work  of  the  miner,  after  he  has 
raised  the  ore  to  the  surface,  to  separate  the  different 
kinds  of  minerals,  in  order  to  prepare  a  product,  or 
products,  which  can  be  dealt  with  by  the  smelter  for 
the  extraction  of  valuable  metal. 

STIBNITE 

(Plate  4,  Figs,  i  and  2). — This  is  a  sulphide  of 
antimony,  with  the  chemical  formula  Sb2S3.     It  is 

78 


STIBNITE  79 


also  often  known  by  the  name  antimonite  or  anti- 
mony-glance, and  it  is  the  most  important  of  the  ores 
of  antimony. 

It  is  usually  found  as  fibrous  or  lamellar  masses, 
which  break  with  shining  flat  surfaces,  owing  to  the 
existence  of  a  perfect  cleavage  in  one  direction  in 
the  crystals.  A  very  characteristic  feature  of  stib* 
nite  is  that  these  bright  cleavage  surfaces  are  marked 
by  fine  striations  in  a  direction  at  right  angles  to 
their  length.  Sometimes  the  fibrous  or  acicular  crys- 
tals, which  build  up  the  massive  material,  have  a 
radial  or  stellate  arrangement,  as  represented  in  Fig.  i. 

Crystals  of  stibnite  belong  to  the  orthorhombic 
system,  and  have  the  form  of  long  prisms,  varying 
from  the  thickness  of  a  needle  to  an  inch  or  more 
across.  They  are  usually  much  grooved  and  fur- 
rowed in  the  direction  of  their  length  (Fig.  2) ;  and 
the  perfect  cleavage  is  also  parallel  to  the  length  of 
the  crystals.  Magnificent  groups  of  prismatic  crys- 
tals from  Japan  are  known;  and  very  good  specimens 
are  abundant  at  Felsobanya,  in  Hungary,  this  being 
the  locality  of  the  specimen  represented  in  Fig.  2, 
and  also  of  that  shown  in  Plate  20,  Fig.  i,  where 
the  needles  of  stibnite  are  associated  with  crystals  of 
barytes. 

The  color  of  the  mineral  is  steel-grey  and  the 
luster  metallic,  but  on  exposure  to  light  the  surface 
gradually  becomes  dulled  with  a  bluish  or  blackish 
tarnish.  The  mineral  is  quite  soft  (hardness  =  2), 
and  the  crystals  are  easily  damaged,  unless  handled 
with  care. 


80          THE    WORLD'S    MINERALS 

Metallic  antimony,  which  for  the  most  part  is 
obtained  from  the  mineral  stibnite,  is  much  used  for 
making  alloys.  For  example,  type-metal  as  used  in 
printing  is  an  alloy  of  four  parts  of  lead  with  one  of 
antimony.  Salts  of  antimony  find  an  application  in 
medicine,  and  as  pigments.  Stibnite  is  used  in  the 
East  for  darkening  the  eyebrows,  and  it  was  also  used 
by  the  ancients  for  the  same  purpose. 

REALGAR 

(Plate  4,  Figs.  3  and  4). — Two  distinct  sulphides 
of  arsenic  occur  in  nature  as  minerals;  they  differ 
not  only  in  the  proportions  in  which  the  arsenic  and 
sulphur  are  chemically  combined,  but  also  in  their 
external  characters,  the  difference  in  color  being  par- 
ticularly striking.  These  two  minerals — the  bright 
red  realgar  and  the  bright  yellow  orpiment — fre- 
quently occur  together,  as  is  shown  in  the  two  pictures 
on  Plate  4. 

In  realgar,  each  chemical  molecule  consists  of  one 
atom  of  arsenic  and  one  atom  of  sulphur,  the  formula 
being  AsS.  This  compound  is  found  in  nature  as 
small,  monoclinic  crystals,  with  an  aurora-red  color, 
and  often  with  perfect  transparency  (Fig.  4).  On 
exposure  to  light,  these  crystals  undergo  a  remark- 
able change;  after  some  time,  in  place  of  a  brilliant 
crystal,  we  find  a  heap  of  yellow  powder.  This 
change  is  due  to  an  absorption  of  oxygen  from  the 
air,  with  the  production  of  the  yellow  sulphide  (As2- 
S3)  and  white  arsenious  oxide  (As2O3).  The  latter 


SULPHIDES. 


Plate  4. 


1,  2,  Stibnite.    3,  4,  Realgar  and  OrpimcnV 


ORPIMENT— MOLYBDENITE          81 

is  soluble  in  water,  and,  like  all  soluble  arsenic  com- 
pounds, it  is  extremely  poisonous.  It  is,  therefore, 
necessary  to  keep  specimens  of  this  mineral  in  the 
dark,  otherwise  they  soon  become  spoilt. 

Both  the  specimens  represented  on  Plate  4  are 
from  Hungary.  In  Fig.  3  the  realgar  is  massive, 
and  shows  no  crystalline  form. 

ORPIMENT 

(Plate  4,  Figs.  3  and  4). — In  the  other  native  sul- 
phide of  arsenic,  two  atoms  of  arsenic  are  combined 
with  three  atoms  of  sulphur  to  form  the  chemical 
molecule,  the  formula  here  being  As2S3,  which  is 
analogous  to  the  formula  of  stibnite,  with  arsenic 
in  place  of  antimony.  The  mineral  also  shows  a  re- 
lation to  stibnite  in  its  crystalline  structure  and  per- 
fect cleavage  in  one  direction.  It  usually  occurs  as 
lamellar  masses,  distinctly  developed  crystals  being 
extremely  rare.  The  color  is  bright  lemon-yellow; 
and  the  bright,  smooth  cleavage  surfaces  have  a 
pearly  appearance.  The  name  orpiment  is  a  corrup- 
tion of  the  Latin  name  auripigmentum,  which  means 
"gold  paint."  The  pigment  known  as  "king's  yellow" 
is,  however,  now  prepared  artificially. 

MOLYBDENITE 

(Plate  5,  Fig:  i). — This  mineral  is  the  disulphide 
of  the  rare  metal  molybdenum,  the  chemical  formula 
being  MoS2.  It  is  found  as  platy  or  lamellar  masses, 


82          THE    WORLD'S    MINERALS 

and  sometimes  as  six-sided  plates  or  scales;  but  dis- 
tinctly formed  crystals  are  rare.  It  is  opaque,  with 
a  metallic  luster  and  a  lead-grey  color.  The  material 
is  very  soft  (hardness  —  i)  and  greasy  to  the  touch, 
and  it  readily  marks  paper.  This  mineral  presents, 
in  fact,  a  most  striking  resemblance  to  graphite  (p. 
63),  with  which  it  is  sometimes  confused.  There  is, 
however,  a  slight  difference  in  color,  which  is  notice- 
able when  the  two  minerals  are  compared  side  by 
side,  molybdenite  showing  a  bluish  tinge.  In  the 
specific  gravity  there  is  a  very  wide  difference,  the 
value  for  molybdenite  being  4.7,  and  for  graphite 
only  2.2.  This  affords  a  ready  means  of  distinguish- 
ing the  two  minerals,  and  the  difference  in  weight 
will  be  at  once  noticed  if  pure  specimens  of  equal 
size  are  handled.  When,  however,  a  small  amount 
of  the  mineral  is  embedded  in  a  large  piece  of  matrix, 
the  specific  gravity  cannot  be  judged  by  merely 
handling  the  specimen.  We  may  then  detach  a  small 
scale  of  the  mineral  and  drop  it  into  a  heavy  liquid 
(such  as  bromoform,  sp.  gr.  2.8 ;  or  methylene  iodide, 
sp.  gr.  3.3)  ;  if  the  flake  be  molybdenite  it  will  sink, 
while  if  it  be  graphite  it  will  float  on  the  surface 
of  the  liquid.  The  chemical  behavior  also  serves  to 
distinguish  the  two  minerals;  when  heated  before 
the  blowpipe,  molybdenite  gives  a  strong  smell  of 
burning  sulphur. 

Molybdenite  is  found  as  scales  embedded  in  vari- 
ous crystalline  rocks,  such  as  gneiss  and  granite;  and 
it  is  also  met  with  in  some  metalliferous  veins.  It  is 
mined  in  Norway,  Canada,  and  New  South  Wales. 


ZINC-BLENDE  83 

The  mineral  is  used  to  a  small  extent  for  the  prep- 
aration of  the  molybdenum  compounds  used  as  re- 
agents in  the  chemical  laboratory.  Within  recent 
years  it  has  been  employed  in  the  manufacture  of 
steel ;  the  addition  of  molybdenum  gives  a  very  tough 
and  hard  steel  especially  suitable  for  tools. 

ZINC-BLENDE 

(Plate  5,  Fig.  2).— The  sulphide  of  zinc  (ZnS), 
known  as  zinc-blende,  or  simply  as  blende,  is  the 
most  abundant  of  zinc-bearing  minerals,  and  at  the 
same  time  the  most  important  ore  of  this  metal. 

Crystals  belong  to  the  tetrahedral  division  of  the 
cubic  system,  and  their  characteristic  form  is  well 
shown  in  the  picture  (Fig.  2).  Each  of  the  three 
crystals  here  represented  on  the  rocky  matrix  con- 
sists of  a  tetrahedron,  the  edges  of  which  are  trun- 
cated by  narrow  faces  of  the  cube,  and  the  corners  are 
truncated  by  small  faces  of  another  (so-called  nega- 
tive) tetrahedron;  in  addition  there  are  six  small  tri- 
angular faces  symmetrically  grouped  round  each  of 
the  corners.  Although  crystals  of  zinc-blende  are  of 
quite  common  occurrence,  it  is  not  often  that  their 
form  can  be  so  easily  made  out  as  in  the  example  just 
mentioned.  More  usually  the  crystals  are  much 
distorted  and  the  faces  rounded,  while  frequent 
twinning  still  further  adds  to  their  complexity.  A 
very  important  crystallographic  character  is  the  ex- 
istence of  perfect  cleavages  in  six  directions  parallel 
to  the  faces  of  the  rhombic-dodecahedron;  the  angles 


84          THE    WORLD'S    MINERALS 

between  adjacent  cleavage  surfaces  is,  therefore, 
120°  or  90°. 

The  faces  of  crystals  and  the  cleavage  surfaces  are 
often  very  smooth  and  bright,  and  they  display  a 
characteristic  resinous  to  adamantine  luster.  The 
color  and  transparency  range  between  wide  limits — 
from  almost  colorless  and  transparent  to  jet-black  and 
opaque.  Most  commonly  the  mineral  is  dark  yellow 
or  brown  and  translucent,  as  represented  in  the  pic- 
ture. This  mineral  is  thus  very  variable  in  its  ap- 
pearance, and  this,  together  with  the  usually  obscure 
crystalline  form,  makes  its  identification  sometimes 
a  matter  of  difficulty.  Indeed,  the  old  German  word 
blende  means  "to  deceive,"  and  the  scientific  name 
sphalerite,  derived  from  the  Greek,  has  the  same  sig- 
nification. The  miners'  names,  "black  jack"  and 
"false  lead,"  also  indicate  that  the  mineral  was  often 
mistaken  for  lead-ore. 

As  a  help  towards  identifying  the  mineral,  it 
should  be  scratched  across  a  bright  cleavage  surface 
with  a  knife  (having  a  hardness  of  rather  less  than  4 
it  readily  yields),  when  a  brown  powder  will  be  ob- 
tained. This,  in  conjunction  with  the  perfect  cleav- 
ages and  the  adamantine  luster,  will  often  serve  to 
distinguish  zinc-blende  from  other  minerals  it  may 
resemble  in  appearance.  But  as  a  confirmatory  test, 
the  mineral  should  also  be  examined  before  the 
blowpipe.  For  this  purpose  a  small  quantity  of  the 
powdered  mineral  is  mixed  with  sodium  carbonate 
and  heated  on  charcoal  in  a  reducing  flame;  the 
sublimate  so  obtained  on  the  charcoal  is  yellow  when 


SULPHIDES. 


Plate  5. 


I,  Molybdenite.    2,  Zink-blende  or  Sphalerite.    3,  4,  Galena. 


ZINC-BLENDE  85 

hot  and  white  when  cold,  and  if  it  be  moistened  with 
cobalt  nitrate  solution  and  again  heated  it  becomes 
bright  green,  indicating  the  presence  of  zinc. 

When  chemically  pure,  zinc-blende  contains  67 
per  cent,  of  zinc,  but  frequently  iron  is  also  present 
to  the  extent  of  i  or  2  per  cent.,  or  even  as  much  as 
1 8  per  cent.  It  is  owing  to  the  presence  of  this 
variable  amount  of  iron  that  the  mineral  varies  so 
widely  in  color  and  transparency.  The  specific 
gravity  of  4.0  is  much  lower  than  that  of  the  lead-ore 
(galena,  sp.  gr.  7.5),  with  which  zinc-blende  is  so 
often  associated;  and  advantage  is  taken  of  this  dif- 
ference in  the  methods  employed  for  the  separation 
of  the  two  minerals  from  the  ore. 

Besides  occurring  as  crystals,  zinc-blende  is  abun- 
dant as  granular  or  compact  masses,  but  in  such 
masses  the  bright  cleavage  surfaces  may  still  be 
recognized. 

Zinc-blende  occurs  in  metalliferous  veins,  together 
with  galena,  calcite,  fluor-spar,  etc.,  and  these  veins 
often  traverse  limestone  rocks.  Good  crystallized 
specimens  are  found  at  many  localities.  The  speci- 
men represented  in  the  picture  (Fig.  2)  is  from  the 
Binnenthal,  in  Switzerland,  where  small  crystals 
usually  occur  singly  in  cavities  of  a  white  dolomite. 

As  an  ore  of  zinc  this  mineral  is  mined  in  Cum- 
berland, Wales,  Germany,  Missouri,  and  many  other 
localities.  Metallic  zinc  is  largely  used  for  gal- 
vanizing iron,  making  alloys  (brass,  etc.),  and  in  the 
construction  of  galvanic  batteries.  Its  salts  are  used 
in  dyeing  and  in  medicine. 


86          THE   WORLD'S    MINERALS 


GALENA 

(Plate  5,  Figs.  3  and  4). — This  is  sulphide  of  lead 
(PbS),  and  a  very  important  ore,  containing  86.6 
per  cent,  of  the  metal.  Curiously,  however,  it  is  also 
of  importance  as  an  ore  of  silver,  since  galena  almost 
invariably  contains  a  minute  proportion  of  silver, 
averaging  about  0.03  per  cent,  or  about  10  oz.  of 
silver  to  the  ton  of  galena.  When  the  ore  is  smelted, 
this  small  amount  of  silver  alloys  with  the  metallic 
lead;  and  before  the  lead  is  sold  for  plumber's  work 
the  silver  is  extracted  from  it  by  a  special  process. 
Large  amounts  of  silver  have  been  obtained  from  the 
lead-roofing  of  old  buildings  by  this  modern  process. 
Good  crystals  of  galena  are  quite  common.  They 
belong  to  the  cubic  system,  and  usually  have  the  form 
of  the  cube,  or  of  the  cube  combined  with  the  octa- 
hedron. In  the  crystals  represented  on  Plate  5,  the 
predominating  form  is  the  octahedron,  the  corners 
of  which  are  truncated  by  small  faces  of  the  cube. 
In  Fig.  3  there  are  also  narrow  faces  of  an  icositet- 
rahedron  (p.  14)  replacing  the  edges  between  the 
cube  and  the  octahedron ;  and  in  Fig.  4  the  edges  of 
the  octahedron  are  truncated  by  narrow  faces  of  the 
rhombic-dodecahedron. 

The  crystals  can  be  split,  or  cleaved,  with  great 
ease  parallel  to  the  faces  of  the  cube,  and  the  surfaces 
of  fracture  are  perfectly  bright  and  smooth.  These 
perfect  cleayages  in  three  directions  at  right  angles 
are  also  shown  by  massive  galena,  on  which  no  ex- 


GALENA— NICCOLITE  87 

ternal  crystal  faces  are  present;  and  any  lump  of 
galena-ore  when  broken  shows  three  sets  of  bright 
steps,  while  isolated  fragments  have  quite  the  ap- 
pearance of  dice.  This  important  character,  taken 
in  conjunction  with  the  lead-grey  color  and  bright 
metallic  luster,  makes  galena  easily  recognizable  at 
sight.  The  softness  (hardness  =  2^)  and  heaviness 
(sp.  gr.  7.5)  are  also  characters  of  importance.  The 
mineral  is  opaque,  and  its  streak  is  lead-grey. 

Galena  commonly  occurs  in  metalliferous  veins 
intersecting  limestone  rocks,  and  is  often  associated 
with  zinc-blende,  quartz,  calcite,  fluor-spar,  etc.  It 
is  of  wide  distribution,  and  is  mined  in  many  parts 
of  the  world;  for  instance,  in  Derbyshire,  County 
Durham,  Lanarkshire,  Spain,  Germany,  and  the 
United  States.  The  two  specimens  represented  in 
Plate  5  are  from  Neudorf ,  in  the  Harz  Mountains ; 
here  the  crystals  are  associated  with  small  rhombo- 
hedra.  of  chalybite  (iron  carbonate)  on  a  base  of 
quartz. 

NICCOLITE 

(Plate  6,  Fig.  i). — A  name  very  often  misapplied 
to  this  mineral  is  copper-nickel,  from  the  old  Ger- 
man name  Kupfernickel;  but,  although  the  mineral 
has  a  very  characteristic  copper-red  color,  it  contains 
no  copper.  It  is,  in  fact,  an  arsenide  of  nickel 
(NiAs).  Crystals  are  very  rare,  and  the  mineral  is 
usually  found  as  compact  masses.  It  is  brittle  and 
breaks  with  an  irregular  fracture.  This  character, 
together  writh  its  greater  hardness  (5^2)  and  black 


88          THE    WORLD'S    MINERALS 

streak,  is  sufficient  to  distinguish  it  from  native  cop- 
per, which  in  color  it  so  closely  resembles. 

Niccolite  is  found,  together  with  ores  of  silver, 
cobalt,  and  copper,  at  several  places  in  Germany, 
and  it  has  also  been  met  with  in  Cornwall  and  Scot- 
land. The  specimen  figured  is  from  the  copper- 
mining  district  of  Mansfeld,  in  Prussia.  Here  the 
mineral  was  formerly  used  for  the  extraction  of 
nickel,  and  also  as  a  source  of  arsenic;  but  most  of 
the  nickel  of  commerce  is  now  obtained  from  pyrrho- 
tite  (p.  94)  and  a  silicate  of  nickel. 

CINNABAR 

(Plate  6,  Figs.  2  and  3). — This  is  sulphide  of 
mercury  (HgS),  and  is  of  importance  as  being  the 
only  ore  of  mercury,  or  quicksilver.  This  remark- 
able metal,  which  at  the  ordinary  temperature  is  a 
liquid  1^/2  times  as  heavy  as  water,  is  also  found  in 
the  native  state,  though  only  in  small  amount,  so  that 
it  is  not  a  mineral  of  any  economic  importance.  It 
is  often  to  be  seen  on  specimens  of  cinnabar,  to  the 
easy  decomposition  of  which  mineral  it  owes  its 
origin.  The  silver-white  spots  on  the  upper  portion 
of  Fig.  3  represent  small  liquid  globules  of  native 
mercury. 

Crystals  of  cinnabar  are  not  very  common,  and  are 
mostly  quite  small.  Large,  well-shaped  rhombo- 
hedra  have,  however,  recently  come  from  the  cin- 
nabar-mines in  central  China;  these  crystals  usually 
consist  of  two  interpenetrating  rhombohedra  inter- 


SULPHIDES  &  ARSENIDES. 


Plate  6. 


a 


tv, 

* '•  **// 


1,  Niccolite.    2,  3,  Cinnabar.    4,  Mispickel  or  Arsenopyrite. 


CINNABAR— MISPICKEL  89 

grown  in  twinned  position.  The  crystals  have  per- 
fect cleavages  parallel  to  the  six  faces  of  the  hex- 
agonal prism.  They  are  often  transparent,  with  a 
deep-red  color  and  brilliant  luster.  On  the  darker 
crystals  the  luster  is  quite  metallic  in  character,  es- 
pecially when  the  full  blaze  of  reflected  light  is  ob- 
served from  one  of  the  crystal  faces  (Fig.  2). 

The  crystals  are  quite  soft  (hardness  =  2),  and 
when  scratched  they  yield  a  scarlet  powder.  This  is 
the  characteristic  color  of  the  mineral  in  its  granular 
or  earthy  condition  as  commonly  seen  in  the  ore.  In 
Fig.  2  there  is  shown  a  single  crystal  of  cinnabar 
on  massive  cinnabar;  and  in  Fig.  3  the  massive  min- 
eral is  scattered  through  the  rocky  matrix,  which 
contains  also  yellow  specks  of  iron-pyrites.  Some- 
times the  ore  is  dark  brown  or  black,  owing  to  the 
admixture  of  clay  or  carbonaceous  material. 

Veins  of  mercury-ore  are  rather  sporadic  in  their 
distribution,  and  mines  are  worked  at  but  few  places, 
notably  Almaden  in  Spain,  Idria  in  Carniola,  and 
New  Almaden  in  California. 

MISPICKEL 

(Plate  6,  Fig.  4). — This  mineral,  which  is  also 
known  as  arsenopyrite,  or  arsenical  pyrites,  is  a  com- 
pound of  iron,  arsenic,  and  sulphur  in  equal  atomic 
proportions,  the  formula  being  FeAsS.  It  is  thus, 
at  the  same  time,  a  sulphide  and  an  arsenide,  and 
may  be  described  as  a  sulph-arsenide  of  iron. 

Crystals  of  mispickel  belong  to  the  orthorhombic 


90  THE    WORLD'S    MINERALS 

system.  Their  characteristic  rhombic  form  is  clearly 
shown  in  the  picture;  here  we  have  a  short  rhombic 
(four-sided)  prism  terminated  at  either  end  by  a 
pair  of  dome  faces  like  the  roof  of  a  house.  Usually 
the  prism  faces  are  longer,  and  the  crystals  are  then 
prismatic  in  habit.  Columnar  and  granular  aggre- 
gates of  massive  mispickel  are  also  of  common 
occurrence. 

The  mineral  is  tin-white,  with  a  metallic  luster, 
but  it  often  shows  a  bluish  (Fig.  4),  yellowish,  or 
blackish  tarnish.  The  specific  gravity  is  6;  and  the 
hardness  is  also  6,  so  that  the  mineral  can  be  scratched 
with  a  knife  only  with  difficulty. 

Mispickel  occurs  in  metalliferous  veins,  with  ores 
of  silver,  copper,  or  tin.  It  is  abundant  in  Cornwall 
and  Devon,  and  at  Freiberg,  in  Saxony.  The  speci- 
men represented  in  Fig.  4  is  from  the  latter  locality; 
the  large  crystals  rest  with  minutely  crystallized 
chalybite  on  a  bed  of  quartz. 

This  mineral  is  of  importance  in  being  the  prin- 
cipal source  of  the  extremely  poisonous  arsenious 
oxide  (As2O3),  or  white  arsenic,  which  is  obtained 
by  simply  roasting  the  ore  in  a  current  of  air. 

A  variety  of  mispickel,  in  which  a  part  of  the  iron 
is  replaced  by  an  equivalent  amount  of  cobalt  (5  to 
10  per  cent),  is  called  danaite,  in  honor  of  the  fa- 
mous American  mineralogist,  whose  several  books 
have  long  been  standard  works  in  this  branch  of 
science. 


MARCASITE 


MARCASITE 

(Plate  7,  Fig.  i). — This  is  disulphide  of  iron 
(FeS2),  like  the  more  common  mineral  iron-pyrites 
to  be  next  described.  These  two  minerals,  although 
identical  in  chemical  composition,  are  quite  distinct 
in  their  crystalline  form  and  physical  characters.  We 
have  here  another  example  of  dimorphism,  just  as 
with  the  two  crystalline  forms  of  carbon  (p.  63). 

Marcasite  also  shows  a  relation  to  mispickel,  the 
mineral  last  considered,  in  that  the  type  of  crystal- 
lization is  the  same  and  the  angles  between  the  faces 
almost  identical.  Further,  if  we  consider  one  of  the 
atoms  of  sulphur  in  the  marcasite  formula  to  be  re- 
placed by  one  atom  of  arsenic,  we  arrive  at  the 
formula  of  mispickel.  The  two  minerals  mispickel 
and  marcasite  are,  therefore,  said  to  be  isomorphous, 
meaning,  in  Greek,  of  the  same  form. 

The  form  of  marcasite  crystals  is,  however,  very 
often  obscured  by  twinning,  and  we  have  complex 
groups  of  crystals  to  which  the  names  cockscomb- 
pyrites  and  spear-pyrites  are  applied.  The  five- 
sided,  crystal  shown  in  Fig.  i  really  consists  of  five 
crystals  united  together  by  twinning;  the  striated 
faces  correspond  to  the  similarly  striated  dome  faces 
of  mispickel. 

The  color  of  marcasite  is  pale  brass-yellow,  with  a 
metallic  luster,  but  it  is  often  obscured  By  surface 
tarnish  and  alteration.  This  mineral  is,  indeed,  par- 
ticularly liable  to  alteration  by  weathering,  the  sul- 
phur being  removed  as  sulphuric  acid  and  the  iron 


92  THE    WORLD'S    MINERALS 

remaining  behind  as  hydrated  oxide  of  iron,  which 
still  preserves  the  original  form  of  the  crystals.  Thus 
the  brassy-looking  marcasite  comes  to  be  replaced 
by  the  rusty  brown  limonite.  The  specific  gravity 
(4.8)  of  marcasite  is  slightly  less  than  that  of  iron- 
pyrites,  and  the  hardness  is  much  the  same  in  the  two 
minerals.  The  color  of  marcasite  is  usually  rather 
paler  than  that  of  iron-pyrites. 

When  distinctly  formed  crystals  are  not  to  be  ob- 
served, as  in  the  commonly  occurring  nodular  masses 
with  internal  radiating  structure,  it  is  often  extremely 
difficult,  and  in  some  cases  impossible,  to  distinguish 
between  marcasite  and  iron-pyrites. 

Though  not  an  uncommon  mineral,  marcasite  is 
far  less  abundant  and  less  widely  distributed  than  is 
iron-pyrites.  It  is  found  in  metalliferous  veins  in 
Derbyshire  and  Cornwall,  and  in  beds  of  brown  coal 
in  Bohemia.  The  specimen  represented  in  Plate 
7  is  from  Dover,  in  England,  and  shows  one  larger 
and  several  smaller  crystals  embedded  in  a  matrix  of 
chalk.  When  found  in  sufficiently  large  amounts, 
marcasite  is  used  for  the  same  purposes  as  iron- 
pyrites. 

IRON-PYRITES 

(Plate  7,  Figs.  2  and  3). — As  already  mentioned 
this  is  disulphide  of  iron  (FeS2),  identical  in  chem- 
ical composition  with  marcasite.  The  names  pyrites 
and  pyrite  are  also  in  common  use  for  this  species. 
It  is  one  of  the  most  widely  distributed  of  minerals, 
being  not  only  abundant  in  metalliferous  veins,  but 


IRON-PYRITES  93 

found  also  as  crystals  embedded  in  rocks  of  all 
kinds,  while  sometimes  it  forms  enormous  bedded 
deposits. 

Crystals  are  very  common,  and  they  usually  have 
the  form  of  small  cubes  (Fig.  2).  On  a  critical  in- 
spection, it  will  be  seen  that  these  cubes  differ  from 
the  cubes  in  which  most  other  minerals  crystallize; 
their  faces  are  each  striated  parallel  to  only  one  edge, 
and  the  striations  on  adjacent  faces  are  at  right  angles 
to  one  another.  The  system  of  crystallization,  though 
cubic,  is  of  a  lower  (pentagonal-dodecahedral)  type 
of  symmetry.  The  pentagonal-dodecahedron  is  also 
a  common  form  of  crystals  of  iron-pyrites  (Fig.  3) ; 
this  solid  is  bounded  by  twelve  five-sided  faces 
(which  are,  however,  not  regular  pentagons).  The 
faces  of  this  form  are  also  striated  on  the  same  plan 
in  three  directions  at  right  angles — namely,  parallel 
to  the  cubic  axes  of  the  crystal,  as  is  clearly  shown 
in  Fig.  3. 

The  color  both  of  the  crystals  and  of  compact 
masses  is  pale  brass-yellow,  with  a  bright  metallic 
luster;  but  the  powder  or  streak  of  the  mineral  is 
dark  greenish-black  or  brownish-black.  The  min- 
eral shows  no  cleavage;  it  is  brittle  and  breaks  with 
an  irregular  fracture.  The  specific  gravity  is  5,  and 
the  hardness  6.  It  can  scarcely  be  scratched  with  a 
knife;  and  when  struck  with  steel  it  gives  sparks, 
due  to  the  burning  of  the  sulphur  in  the  detached 
fragments.  Iron-pyrites  was  formerly  used  in  tinder- 
boxes,  and  firearms,  and,  indeed,  the  Greek  word 
pyrites  means  "fire  stone."  These  characters  at  once 


94          THE    WORLD'S    MINERALS 

serve  to  distinguish  iron-pyrites  from  native  gold, 
for  which  it  may  possibly  be  mistaken. 

Like  marcasite,  iron-pyrites  is  readily  altered  by 
weathering;  and  pseudomorphs  of  limonite  (hy- 
drated  oxide  of  iron)  with  the  form  of  crystals  of 
iron-pyrites  are  not  at  all  uncommon. 

Iron-pyrites  often  contain  small  amounts  of  copper, 
gold,  and  other  metals.  The  large  deposits  which 
are  extensively  worked  at  Rio  Tinto  in  the  south  of 
Spain,  in  Norway,  and  in  the  Harz  Mountains,  con- 
tain on  the  average  3  per  cent,  of  copper;  but  this 
may  be  accounted  for  by  the  admixture  of  copper- 
pyrites  with  the  iron-pyrites.  Such  ores  are  roasted 
in  a  current  of  air  to  yield  sulphur  dioxide  for  the 
manufacture  of  sulphuric  acid  (oil  of  vitriol),  and 
the  residue  is  then  treated  for  the  extraction  of  the 
copper  and  traces  of  gold. 

Good  crystallized  specimens  of  iron-pyrites  are 
found  at  many  places,  amongst  which  may  be  men- 
tioned the  iron  (magnetite)  mines  of  Traversella,  in 
Piedmont,  Italy,  and  the  iron  (haematite)  mines  of 
Rio,  in  the  island  of  Elba.  The  specimen  represented 
in  Plate  7,  Fig.  3,  is  from  the  latter  locality,  and 
shows  the  crystals  of  iron-pyrites  on  haematite;  that 
in  Fig.  2  is  from  Cornwall. 

PYRRHOTITE 

(Plate  7,  Fig.  4) .— Pyrrhotite,  or  magnetic  pyrites, 
is  another  sulphide  of  iron,  but  one  approximating 
in  composition  to  the  monosulphide  FeS.  Hexagonal 


SULPHIDES. 


Plate  7. 


1,  Marcasite.    2,  3,  Iron-pyrites.    4,  Pyrrhotkei 


PYRRHOTITE— COPPER-PYRITES    95 

crystals  of  platy  habit  are  rare,  and  usually  only 
compact  masses  (Fig.  4)  are  found.  The  color  is 
bronze-yellow  tarnishing  to  brown,  while  the  metallic 
luster  is  usually  dull.  The  mineral  is  rather  softer 
(hardness  =  4)  than  iron-pyrites,  and  it  also  differs 
from  this  in  its  magnetic  character,  small  fragments 
being  attracted  by  a  magnet. 

Pyrrhotite  occurs  as  grains,  embedded  in  certain 
igneous  rocks,  such  as  gabbro,  and  sometimes  it  is 
concentrated  towards  the  margins  of  the  rock-mass, 
forming  workable  deposits  of  ore.  It  is  also  found 
in  metalliferous  veins.  The  ore  sometimes  contains 
a  small  amount  of  nickel  (3  to  5  per  cent.),  as  in  the 
large  deposits  at  Sudbury  in  Ontario,  and  in  Norway, 
where  it  is  largely  worked  for  the  nickel.  The  speci- 
men shown  in  Plate  7  is  from  Bodenmais,  in  Bavaria. 

COPPER-PYRITES 

(Plate  8,  Figs,  i  and  2). — Copper-pyrites,  or 
chalcopyrite,  is  a  sulphide  of  iron  and  copper,  with 
the  formula  CuFeS2,  and  contains,  when  pure,  34^ 
per  cent,  of  copper  and  30^2  per  cent,  of  iron.  It  is 
the  most  abundant  of  copper  minerals,  and  at  the 
same  time  the  most  important  ore  of  copper. 

Crystals  of  copper-pyrites  belong  to  the  tetragonal 
system;  they  are  usually  small  and  indistinct,  and  it 
is  not  often  that  their  form  can  be  easily  deciphered; 
often  the  form  approximates  to  an  octahedron  (Fig. 
2)  or  tetrahedron.  Compact  masses  are  much  more 
common. 


96          THE    WORLD'S    MINERALS 

This  mineral  closely  resembles  iron-pyrites  in 
general  appearance;  it  differs,  however,  in  its 
darker  brass-yellow  color  and  in  its  lower  degree 
of  hardness  (H.  =  4).  Copper-pyrites  can  readily 
be  scratched  with  a  knife,  giving  a  dark,  greenish- 
black  streak,  while  iron-pyrites  is  scratched  only 
with  difficulty.  In  case  of  doubt,  a  confirmatory  test 
should  be  made  by  dissolving  a  little  of  the  powdered 
mineral  in  nitric  acid,  when  the  presence  of  copper 
will  be  indicated  by  the  green  color  of  the  solution. 
On  the  addition  of  a  large  quantity  of  ammonia  to 
this  solution  the  green  color  changes  to  blue,  and 
the  iron  is  thrown  down  as  a  bulky  reddish-brown 
precipitate. 

The  surface  of  massive  copper-pyrites  very  often 
displays  brilliant  iridescent  colors,  such  as  intense 
red,  or  golden  yellow,  which,  combined  with  the 
metallic  luster,  give  the  mineral  a  very  striking  ap- 
pearance. On  this  account  it  is  called  "peacock-ore" 
by  the  miners.  These  iridescent  colors  are  due  to 
an  extremely  thin  film  of  some  alteration  product  on 
the  surface  of  the  copper-pyrites. 

This  mineral  is  found  in  metalliferous  veins,  and 
especially  those  near  the  junction  of  granite  and 
slates;  associated  minerals  are  zinc-blende,  quartz, 
chalybite,  etc.  It  also  forms  masses  in  igneous  rocks, 
as  in  Tuscany;  or  beds  in  sedimentary  rocks,  as  in 
Thuringia.  The  crystallized  specimens  shown  in 
Plate  8  are  from  Cornwall  (Fig.  i,  on  quartz),  and 
Freiberg,  Saxony  (Fig.  2). 


SMALTITE  97 


SMALTITE 

(Plate  8,  Fig.  3). — This  important  ore  of  cobalt 
is  an  arsenide  of  cobalt,  CoAs2,  but  it  also  contains 
variable  amounts  of  iron  and  nickel. 

The  crystals  are  cubic,  but  are  rare,  and  never 
very  sharply  developed.  In  Fig.  3  is  shown  a  con- 
fused group  of  cubes,  with  rounded  surfaces,  and 
the  corners  truncated  by  small  triangular  faces  of 
the  octahedron.  Granular  and  compact  masses  are 
more  usual.  The  color  is  steel-grey,  with  a  metallic 
luster. 

The  presence  of  cobalt  in  this  mineral  is  very  often 
betrayed  by  the  occurrence  of  crimson  specks  and 
patches  of  earthy  erythrite,  or  cobalt-bloom,  on  the 
surface  of  the  specimens.  This  is  a  hydrated  arsenate 
of  cobalt,  formed  by  the  weathering  of  the  smaltite. 
Similarly,  on  the  surface  of  chloanthite  (NiAs2) — a 
mineral  closely  allied  to  and  very  similar  in  appear- 
ance to  smaltite,  but  differing  from  it  in  containing 
nickel  in  place  of  cobalt — there  may  very  often  be 
detected  patches  of  earthy,  apple-green  annabergite, 
or  nickel-bloom.  Indeed,  the  presence  of  these 
alteration  products  affords  a  ready  means  of  recog- 
nizing these  minerals. 

Smaltite  occurs  in  metalliferous  veins,  together 
with  ores  of  silver;  it  is  found  at  several  places  in 
Germany,  notably  at  Schneeberg,  in  Saxony,  this  be- 
ing the  locality  of  the  specimen  represented  in  Plate 
8.  Recently,  large  quantities  of  the  mineral  have 


98          THE    WORLD'S    MINERALS 

been  found  at  Cobalt  City,  near  Lake  Temiskaming, 
in  Ontario.  This  ore  of  cobalt  is  principally  used 
for  the  preparation  of  smalt  (hence  the  name  smalt- 
ite),  or  cobalt-blue,  which  is  employed  as  a  pigment 
and  for  coloring  glass  and  porcelain. 

TETRAHEDRITE 

(Plate  8,  Fig.  4). — Other  names  for  this  mineral 
are  grey-copper-ore  and  fahlore  (German,  Fahlerz) . 
Ideally,  it  is  a  compound  of  copper,  antimony,  and 
sulphur — that  is,  a  sulph-antimonite  of  copper,  with 
the  formula  Cu3SbS3.  Actually,  however,  the  copper 
may  be  in  part  replaced  by  silver,  iron,  zinc,  mer- 
cury, etc.,  and  the  antimony  by  arsenic.  In  certain 
mining  districts  it  is  an  important  ore  of  copper;  but 
when  silver  is  also  present  (sometimes  to  the  extent 
of  30  per  cent),  it  is  of  far  greater  importance  as 
an  ore  of  silver,  and  many  of  the  rich  Bolivian  silver- 
ores  are  of  this  type. 

Its  crystals  belong  to  the  tetrahedral  division  of 
the  cubic  system,  the  name  tetrahedrite  referring,  in 
fact,  to  their  characteristic  tetrahedral  habit  (Fig.  4) . 
In  this  picture  the  predominating  form  of  the  two 
crystals,  shown  attached  to  the  matrix,  is  a  three- 
faced  tetrahedron;  the  acute  corners  are  each  re- 
placed by  three  small  faces  of  the  rhombic-dodeca- 
hedron, and  the  edges  are  truncated  by  narrow  faces 
of  a  deltoid-dodecahedron. 

The  color  is  iron-black  or  steel-grey,  and  the  luster 
metallic.  Sometimes,  however,  the  crystals  have  a 


Plate  8 


6 

1,  2,  Chalcopyrite.     3,  Smaltite.    4,  Tetrahedrite.     5.  Pyrargyrite     6,  Proustite. 


TETRAHEDRITE— PYRARGYRITE    99 

thin  coating  of  copper-pyrites  on  the  surface,  having 
then  the  appearance  of  a  brassy-yellow  mineral.  Fine 
crystals  of  this  kind,  often  with  a  brilliant  iridescent 
tarnish  on  their  surface,  were  formerly  found  in 
Herodsfoot  mine,  Cornwall.  Other  localities  for 
well-crystallized  specimens  are  in  the  Harz  Moun- 
tains (Fig.  4),  and  Kapnik,  in  Hungary. 

PYRARGYRITE 

(Plate  8,  Fig.  5). — This  mineral  is  also  known 
as  dark-red  silver-ore,  or  as  ruby-silver;  and  con- 
taining 60  per  cent,  of  silver,  it  is  a  rich  ore  of  this 
metal.  It  is  a  compound  of  silver,  antimony,  and 
sulphur — that  is,  a  sulph-antimonite  of  silver,  with 
the  formula  Ag3SbS3. 

The  crystals  are  rhombohedral,  and  usually  consist 
of  a  hexagonal  prism,  terminated  by  a  flat  rhombo- 
hedron  (as  shown  in  Fig.  6  for  proustite)  ;  in  Fig.  5 
the  hexagonal  prism  is  terminated  by  large  faces  of  a 
scalenohedron,  with  small  faces  of  the  rhombohedron 
at  the  apex,  the  form  here  being  the  same  as  in  calcite 
(Plate  1 6,  Fig.  i). 

The  color  seen  on  the  surface  of  the  crystals  is 
greyish-black,  but  small  crystals  and  fragments  are 
deep  ruby- red  by  transmitted  light;  and  the  same 
ruby-red  color  is  also  to  be  seen  on  portions  of  the 
crystals  that  are  bruised  or  fractured.  The  color  of 
the  streak,  or  of  the  powdered  mineral,  is  purplish- 
red. 

Pyrargyrite  is  found  in  veins  of  silver-ore  at  An- 


100        THE   WORLD'S    MINERALS 

drcasberg,  in  the  Harz  Mountains;  at  Freiberg,  in 
Saxony;  and  at  Guanajuato,  in  Mexico. 

PROUSTITE 

(Plate  8,  Fig.  6). — This  is  another  ruby  silver-ore, 
known  as  light-red  silver-ore,  which  differs  from  the 
last  in  containing  arsenic  in  place  of  antimony,  the 
chemical  formula  being  Ag3AsS3.  The  pure  mineral 
contains  65^  per  cent,  of  silver.  The  crystals  are 
of  the  same  type  as  those  of  pyrargyrite,  though  the 
scalenohedral  habit  (Fig.  5)  is  more  characteristic 
of  proustite  than  the  prismatic  habit  (Fig.  6).  The 
crystals  are  transparent,  with  a  magnificent  ruby-red 
color;  but  on  exposure  to  light  they  very  soon  blacken 
and  become  opaque.  For  this  reason  specimens  must 
be  kept  in  the  dark.  The  color  of  the  streak  is  scarlet. 

This  difference  in  the  color  of  the  streak  affords  a 
ready  means  of  distinguishing  pyrargyrite  and 
proustite,  which  are  often  confused.  Indeed,  it  seems 
probable  that  Figs.  5  and  6  of  Plate  8  have  been 
accidentally  interchanged,  as  suggested  by  the  habit 
of  the  crystals. 

Magnificent  groups  of  transparent  ruby-red  crys- 
tals of  proustite  are  found  in  the  silver-mines  at 
Chanarcillo,  in  Chile,  and  good  specimens  are  also 
known  from  Freiberg,  in  Saxony. 


CHAPTER  VII 

THE  HALOIDS 

THE  chemical  elements  fluorine,  chlorine,  bromine, 
and  iodine  are  known  collectively  as  halogens,  and 
the  compounds,  or  salts,  formed  by  their  union  with 
a  metal  are  known  as  haloids.  This  word  is  derived 
from  the  Greek  name  for  salt — that  is,  common  salt  or 
rock-salt,  which  may  be  taken  as  the  type  of  this 
group. 

Only  a  small  number  of  minerals  fall  into  this 
group,  and  most  of  them  are  quite  rare  and  not  suit- 
able for  illustration  in  color.  To  the  descriptions  of 
the  three  species  figured  in  Plates  8  and  9,  we  shall 
add  a  brief  mention  of  one  or  two  others  which 
present  certain  points  of  interest. 

In  their  general  appearance  and  external  charac- 
ters the  different  minerals  of  this  group  have  little  in 
common,  though  they  are  all  transparent  to  a  greater 
or  less  degree.  In  color  they  exhibit  a  wide  range, 
and  their  practical  applications  are  equally  diverse. 

FLUOR-SPAR 

(Plate  9,  Figs,  i  and  2). — Fluor,  or  fluorite,  is  a 
beautiful  mineral,  presenting  many  points  of  interest. 
It  is  of  fairly  wide  distribution,  and  in  certain  dis- 

101 


102        THE;,  WORLD'S    MINERALS 

tricts  it  is  found  in  enormous  quantities  in  association 
with  lead-ore. 

Chemically,  it  is  a  fluoride  of  calcium  (CaF2), 
being  a  compound  of  the  extremely  active  gas  fluo- 
rine with  the  metal  calcium.  It  is  insoluble  in  water, 
and  is  very  resistant  to  most  chemical  reagents;  but 
when  warmed  with  sulphuric  acid,  it  is  decomposed 
with  the  liberation  of  hydrofluoric  acid  gas.  This 
gas  readily  attacks  glass  and  other  silicates,  and  on 
this  action  depends  its  use  for  etching  glass.  Fluor- 
spar fuses  at  a  red-heat,  and  on  this  account  it  is 
employed  as  a  flux  in  the  smelting  of  ores.  For  this 
reason,  the  mineral  takes  its  name  from  the  Latin 
fluof  I  flow. 

Good  crystallized  specimens  are  common.  The 
crystals  belong  to  the  cubic  system,  and  most  fre- 
quently have  the  form  of  perfect  cubes  (Plate  9) .  On 
specimens  from  certain  localities  the  edges  of  the 
cube  are  sometimes  each  bevelled  by  two  narrow 
faces  of  a  form  known  as  a  four-faced  cube;  or 
again,  the  corners  may  be  each  replaced  by  six  small 
faces  of  a  six-faced  octahedron;  but  these  modifica- 
tions of  the  cube  are  comparatively  rare.  Another, 
but  far  less  common  form,  is  the  simple  octahedron, 
as  in  the  beautiful  pink  octahedra  of  the  fluor-spar 
from  Chamounix  and  a  few  other  places  in  the  Alps. 

Very  frequently  the  crystals  are  twinned;  there 
being  an  intergrowth  or  interpenetration  of  two 
cubes,  with  the  result  that  the  corners  of  one  cube 
project  from  the  faces  of  the  other.  The  two  cubes 
are  related  to  one  another  in  a  perfectly  regular 


HALOIDS. 


Plate  9. 


1,  2,  Fluorite  or  Fluor-spar. 


FLUOR-SPAR  103 


geometrical  manner;  one  of  the  diagonals  joining 
opposite  corners  of  the  cube  is  common  to  the  two 
cubes ;  and  if  one  of  the  cubes  be  rotated  about  this 
diagonal  as  axis  through  half  a  complete  revolution, 
it  will  come  into  a  position  coincident  with  the  sec- 
ond cube.  This  peculiar  twin  intergrowth  is  an 
extremely  characteristic  feature  of  fluor-spar. 

Another  very  important  character  of  this  mineral 
is  its  perfect  octahedral  cleavage — that  is,  the  crystals 
may  be  split  with  ease  along  four  plane  directions 
parallel  to  the  faces  of  the  octahedron.  For  this  rea- 
son specimens  of  fluor-spar  are  very  liable  to  be 
damaged  by  having  the  projecting  corners  of  the 
cubes  knocked  off.  Thus,  in  Plate  9,  Fig.  i,  some 
of  the  corners  of  the  cubes  are  replaced  by  small 
triangular  cleavage  surfaces.  In  the  same  specimen 
may  be  seen  cleavage  cracks  in  the  interior  of  the 
crystals,  with  the  cracks  intersecting  the  cube  faces 
parallel  to  the  diagonal  of  the  latter. 

The  faces  of  the  crystals  and  of  the  cleavage  sur- 
faces are  usually  very  bright  and  smooth,  with  a 
luster  like  that  of  glass.  Fluor-spar  can  be  readily 
scratched  with  a  knife;  and  it  is  very  brittle.  Its 
specific  gravity  is  3.2. 

The  range  of  colors  shown  by  fluor-spar  is  very 
extensive,  and,  indeed,  no  other  mineral  affords  so 
good  an  example  of  how  color  may  vary  in  one  and 
the  same  species.  When  quite  pure,  fluor-spar  is 
perfectly  colorless  and  transparent.  The  mineral 
may  also  show  delicate  or  intense  shades  of  blue, 
purple  (Fig.  i),  green  (Fig.  2),  pink,  yellow,  or 


104        THE    WORLD'S    MINERALS 

sometimes  even  black.  Frequently  also  the  same  crys- 
tal may  be  of  different  colors  in  layers  parallel  to  the 
cube  faces.  The  well-known  "blue-John"  of  Derby- 
shire is  a  dark  purplish,  fibrous  variety  of  fluor-spar. 
These  colors  are  due  to  the  presence  of  mere  traces 
of  coloring  matter,  so  small  in  amount  that  the  exact 
nature  of  the  pigment  is  still  a  matter  for  discussion. 
The  fact  that  in  many  cases  the  color  is  destroyed  by 
heat — the  fluor-spar  becoming  quite  colorless — sug- 
gests that  the  coloring  matter  is  a  hydrocarbon  of 
some  kind.  Certain  specimens  are  changed  in  color 
on  exposure  to  sunlight;  for  example,  green  may  be 
changed  to  purple. 

A  remarkable  property  exhibited  by  some  crystals 
of  fluor-spar — particularly  those  from  the  north  of 
England — is  that  they  show  one  color  by  transmitted 
light,  and  another  color  by  reflected  light.  The 
crystals  like  those  represented  in  Fig.  i,  if  held  be- 
tween the  observer's  eye  and  the  window,  appear 
pale  green;  but  if  the  eye  be  between  the  specimen 
and  the  window,  a  rich  velvety,  plum-blue  color  is 
to  be  seen  on  the  surfaces  of  the  crystals.  This  phe- 
nomenon, having  been  first  recognized  in  fluor-spar, 
is  known  as  fluorescence.  Another  interesting  prop- 
erty, of  a  somewhat  similar  optical  nature,  is  that 
of  phosphorescence.  When  fluor-spar  is  heated  to  a 
temperature  below  red-heat  it  glows  with  a  soft 
greenish  light,  like  that  of  the  glow-worm. 

Fluor-spar  occurs  in  nature  under  a  variety  of  con- 
ditions. It  is  found  in  granite,  and  in  veins  of  tin-ore, 
in  Cornwall;  in  gneiss  in  the  Alps;  in  limestone,  and 


FLUOR-SPAR— ROCK-SALT          105 

in  veins  of  lead-ore,  in  Derbyshire  and  the  north  of 
England;  and  rarely  in  the  lava  of  Vesuvius. 

The  best  crystallized  specimens  are  those  found 
abundantly  in  the  lead-mines  of  the  north  of  Eng- 
land, in  Cumberland,  Northumberland,  and  more 
particularly  in  County  Durham.  In  Weardale 
(County  Durham)  large  cavities  completely  lined 
with  beautiful  crystals,  measuring  sometimes  as  much 
as  twelve  inches  or  even  more  along  the  edges  of 
the  cubes,  are  not  uncommonly  met  with.  The  two 
specimens  represented  in  Plate  9  are  from  Weardale; 
in  Fig.  i  the  crystals  are  partly  encrusted  with  chaly- 
bite,  and  in  Fig.  2  they  rest  on  a  base  of  iron-stone. 

Some  of  the  practical  uses  of  fluor-spar  have  al- 
ready been  briefly  mentioned.  Large  quantities — 
thousands  of  tons  annually — are  mined  in  Derbyshire 
and  the  north  of  England  for  use  as  a  flux  in  smelting 
ores.  The  mineral  is  also  used  for  the  preparation 
of  hydrofluoric  acid  and  for  etching  glass,  and  in  the 
manufacture  of  opaline  glass.  Perfectly  colorless  and 
transparent  crystals  find  an  application  in  the  con- 
struction of  lenses  for  microscopes ;  while  the  Derby- 
shire "blue-John"  is  carved  into  vases  and  other  small 
ornaments.  The  finer  colored  crystals  have  occasion- 
ally been  cut  as  gem-stones,  but  the  low  degree  of 
hardness  of  the  mineral  renders  it  unsuitable  for  use 
in  jewelry. 

ROCK-SALT 

(Plate  10,  Figs,  i  and  2). — Salt,  or  common  salt, 
is  a  mineral  of  great  importance  in  everyday  life. 


106        THE    WORLD'S    MINERALS 

To  the  mineralogist  it  is  known  as  rock-salt,  or  halite 
(from  the  Greek  name  for  salt) .  The  reddish-brown 
lumps  of  salt  given  to  cattle  to  lick  is  rock-salt  in  the 
condition  in  which  it  is  found  in  the  earth.  This  may 
be  readily  purified  by  dissolving  it  in  water  and  al- 
lowing the  earthy  impurities  to  settle;  on  evaporating 
the  clear  solution,  the  pure  white  table-salt  or  cook- 
ing-salt is  obtained.  It  may  be  remarked  in  passing 
that  very  few  minerals  are  soluble  in  water;  the 
only  one  described  in  this  book  being  the  one  under 
discussion. 

Salt  is  deposited  by  the  evaporation  of  sea-water 
in  land-barred  gulfs  and  from  the  salt  water  of  in- 
land lakes,  such  as  the  Dead  Sea  and  the  Great  Salt 
Lake  of  Utah.  Such  deposits  may  become  covered 
up  by  layers  of  mud,  and  so  protected  and  preserved. 
In  this  way  beds  of  rock-salt,  sometimes  of  enormous 
thickness,  have  been  formed  in  past  epochs  of  the 
earth's  history;  and  these  are  now  found  and  mined 
in  the  sedimentary  rocks  of  various  geological  pe- 
riods. The  beds  of  rock-salt  for  which  Cheshire  is 
famous  are  interstratified  with  the  red  marls  and 
standstones  of  Triassic  age,  while  many  of  those  in 
the  United  States  are  of  considerably  greater  an- 
tiquity. 

The  rock-salt  occurring  in  these  beds  is  massive, 
and  may  be  quite  pure,  being  then  white  or  colorless, 
or  it  may  be  somewhat  intermixed  with  clay  and  red 
oxide  of  iron,  when  it  is  reddish-brown  in  color. 
Cavities  when  present  in  this  massive  material  are 
lined  with  crystals  of  rock-salt.  Good  crystals  are, 


ROCK-SALT  107 


however,  not  common ;  the  best  come  from  Strassfurt, 
in  Prussia  (Plate  10,  Fig.  i),  and  from  Wieliczka,  in 
Poland  (Fig.  2). 

Crystals  of  rock-salt  belong  to  the  cubic  system, 
and  almost  invariably  they  have  the  form  of  simple 
cubes  (Plate  10,  Figs,  i  and  2).  These  crystals  are 
thus  of  exactly  the  same  form  as  those  of  fluor-spar, 
but  there  is  a  very  important  difference  between  them 
in  their  cleavage.  While  in  fluor-spar  the  perfect 
cleavages  are  parallel  to  the  faces  of  the  octahedron, 
in  rock-salt  they  are  parallel  to  the  faces  of  the  cube. 
A  crystal  of  rock-salt  may  therefore  be  easily  broken 
up  into  a  number  of  smaller  cubes,  and  this  sub- 
division can  be  repeated  indefinitely. 

The  crystals  are  usually  perfectly  colorless  and 
transparent  (Fig.  2),  but  occasionally  they  are  of  a 
deep  blue  color.  The  cause  of  this  blue  color  is  very 
mysterious,  for  the  color  disappears  when  the  salt 
is  dissolved  in  water  or  when  it  is  heated;  further, 
a  similar  color  is  induced  in  colorless  rock-salt  by 
heating  it  in  the  vapor  of  metallic  sodium. 

Beds  of  rock-salt  are  sometimes  worked  by  the 
ordinary  methods  of  mining,  but  more  frequently 
water  is  admitted  through  bore-holes  into  the  salt- 
bearing  strata  and  the  solution  of  salt  pumped  up. 
Large  quantities  of  salt  are  also  yielded  by  natural 
salt  springs  and  by  the  solar  evaporation  of  sea-water 
in  salt-pans.  The  famous  mines  of  Wieliczka,  near 
Cracow,  which  have  been  worked  for  the  last  600 
years,  form  a  large  subterranean  town  carved  in  the 
thick  bed  of  solid  rock-salt. 


108        THE    WORLD'S    MINERALS 

Some  other  modes  of  occurrence  of  rock-salt  are 
of  interest,  though  of  no  practical  importance.  The 
mineral  is  sometimes  to  be  found  as  an  encrustation 
on  Vesuvian  lava;  and  in  the  extremely  minute  cav- 
ities in  the  quartz  of  many  granites,  cubes  of  rock- 
salt  immersed  in  a  liquid  may  be  seen  under  the 
higher  powers  of  the  microscope. 

ATACAMITE 

(Plate  10,  Fig.  3). — This  is  a  rare  copper  mineral, 
though  at  one  or  two  places  it  is  found  in  sufficient 
abundance  to  be  mined  as  an  ore  of  copper.  It  is  a 
combination  of  chloride  of  copper  and  hydroxide  of 
copper,  with  the  somewhat  complex  formula  CuCU.- 
3Cu(OH)2.  Its  crystals  are  of  a  rich,  deep-green 
color,  with  a  brilliant  luster;  they  have  the  form  of 
rhombic  prisms,  with  pyramidal  terminations.  Small 
but  brilliant  crystals  are  found  in  the  Desert  of  Ata- 
cama  (hence  the  name  atacamite)  and  other  parts 
of  Chile;  while  large  crystals  are  abundant  at  Wal- 
laroo, in  South  Australia.  The  specimen  represented 
in  the  figure  is  from  the  latter  locality. 

Atacamite  was  put  to  a  curious  use  before  the 
days  of  blotting-paper,  the  powdered  mineral  being 
sprinkled  over  letters  to  dry  up  the  ink.  A  sample 
of  the  "writing  sand"  from  Atacama  was  presented 
to  the  British  Museum  collection  by  the  Abbe 
Rochon  in  the  year  1790. 


HALOIDS :  OXID 


Plate  10. 


1,  2,  Halite.    3,  Atacamite.    4,  5,  OpaK 


CERARGYRITE— CRYOLITE        109 


CERARGYRITE 

Or  Horn-silver. — Under  this  term  are  included 
several  minerals  which  are  compounds  of  silver  with 
chlorine,  bromine,  and  iodine.  In  Chile,  Nevada, 
and  New  South  Wales  they  are  found  in  considerable 
quantity  and  are  of  importance  as  ores  of  silver. 
These  minerals  present  a  horn-like  appearance,  and 
like  horn  they  can  be  cut  with  a  knife.  Their  pale 
yellow  or  greenish  color  quickly  darkens  on  exposure 
to  light.  Their  cubic  crystals  are  small  and  obscure, 
and  not  suitable  for  illustration. 

CRYOLITE 

This  is  the  only  other  haloid  that  we  need  mention 
in  this  place.  It  is  a  fluoride  of  sodium  and  alu- 
minium (Na3AlF6) ,  and  is  of  importance  as  an  ore  of 
metallic  aluminium.  It  is  a  white  or  colorless  min- 
eral, with  somewhat  the  appearance  of  ice,  and  for 
this  reason  it  is  known  as  ice-spar  (the  name  cryolite 
having  the  same  meaning  in  Greek) .  It  is  extensive- 
ly mined  at  one  spot  in  Greenland,  and  is  employed 
in  the  manufacture  of  opal-glass  and  iron-enamel. 
Formerly  metallic  aluminium  was  extracted  exclu- 
sively from  cryolite,  but  now  bauxite,  together  with 
a  small  proportion  of  cryolite,  is  employed. 


CHAPTER  VIII 

THE  OXIDES 

THIS  group  of  oxides,  or  compounds  of  oxygen  with 
another  chemical  element,  includes  several  important 
minerals,  some  of  which  are  used  as  precious  stones 
and  others  as  ores  of  the  metals. 

QUARTZ 

(Plates  ii  and  12). — Quartz  is  the  most  abundant 
and  widely  distributed  of  all  minerals.  The  sand 
of  the  seashore  consists  almost  entirely  of  grains  of 
quartz;  and  when  these  are  cemented  together  to 
form  solid  rock,  we  have  the  abundantly  occurring 
sandstone.  Various  other  kinds  of  rocks,  such  as 
granite  and  gneiss,  also  consist  in  part  of  quartz,  and, 
indeed,  the  mineral  is  the  most  important  of  the  rock- 
forming  minerals.  In  the  neighborhood  of  London, 
and  other  places  in  the  southeast  of  England,  prac- 
tically the  only  kind  of  stone  to  be  found  is  flint, 
which  is  a  compact  variety  of  quartz. 

Chemically,  quartz  is  the  dioxide  of  the  non- 
metallic  element  silicon;  this  oxide  is  usually  known 
simply  as  silica,  and  its  formula  is  SiO2. 

Not  only  is  quartz  the  most  abundant  of  minerals, 

110 


QUARTZ  111 


but  it  is  one  that  appears  in  a  greater  variety  of  forms 
or  guises  than  any  other  mineral.  It  may  be  either 
perfectly  colorless  and  transparent  like  glass,  or  black 
and  opaque  like  jet.  Or,  combined  with  varying 
degrees  of  transparency,  it  may  be  of  all  shades  of 
color  from  one  end  of  the  spectrum  to  the  other — 
red,  orange,  yellow,  green,  rarely  blue,  and  violet. 
Again,  in  the  character  of  its  luster,  it  may  be  either 
glassy  or  waxy,  or  quite  dull  and  earthy.  Further, 
it  may  be  crystallized  in  beautiful  forms,  or  it  may 
be  compact  with  no  obvious  crystalline  structure. 

Owing  to  this  great  diversity  in  its  external  ap- 
pearance, a  considerable  number  of  varieties  of 
quartz  are  recognized  and  distinguished  by  special 
names — for  example,  rock-crystal,  amethyst,  jasper, 
carnelian,  agate,  onyx,  flint,  hornstone,  and  many 
others.  These  several  varieties  may  be  conveniently 
divided  into  two  great  groups — namely,  crystallized 
quartz  and  crypto-crystalline  (i.e.  minutely  crystal- 
line) quartz,  which  are  illustrated  on  Plates  11  and 
12  respectively. 

Quartz  is  thus  a  mineral  not  always  easy  to  recog- 
nize. Characters  of  the  first  importance  which  help 
in  its  determination  are  ( i )  the  absence  of  cleavage, 
(2)  the  hardness,  and  (3)  the  specific  gravity.  The 
degree  of  hardness  (No.  7  on  the  scale)  is  such  that 
the  mineral  cannot  be  scratched  with  a  knife,  and 
when  an  attempt  is  made  to  scratch  it  a  metallic  mark 
is  left  on  the  stone.  It  is,  further,  a  comparatively 
light  mineral  (sp.  gr.  2.65) ,  being  only  about  two  and 
a  half  times  as  heavy  as  an  equal  volume  of  water. 


112        THE    WORLD'S    MINERALS 

If,  therefore,  we  have  a  stone,  consisting  to  all  ap- 
pearances of  the  same  kind  of  material  throughout, 
which  is  not  heavy,  cannot  be  scratched  with  a  knife, 
and  shows  no  trace  of  cleavage,  we  are  fairly  safe 
in  pronouncing  that  it  consists  of  the  very  common 
mineral  quartz.  By  way  of  confirmation,  we  may 
determine  whether  a  fragment  of  the  stone  remains 
suspended  together  with  a  known  crystal  of  quartz 
in  a  heavy  liquid  (such  as  a  mixture  of  methylene 
iodide  and  benzine;  see  p.  40)  of  suitable  density. 

If,  however,  the  specimen  is  crystallized,  there 
need  not  be  the  slightest  hesitation  in  the  determina- 
tion, for  crystals  of  quartz  possess  an  extremely  char- 
acteristic form.  As  clearly  shown  in  Plate  n,  Figs. 
1-3,  they  consist  of  a  hexagonal,  or  a  regular  six- 
sided,  prism  capped  by  a  six-sided  pyramid.  In  Fig. 
2  the  six  pyramid  faces  are  of  equal  size,  but  in  Figs. 
i  and  3  they  are  alternately  larger  and  smaller,  with 
a  three-fold  arrangement.  Additional  faces  are 
sometimes,  though  rarely,  present  on  crystals  of 
quartz.  On  each  of  the  three  alternate  corners  be- 
tween the  prism  and  pyramid  there  may  occasionally 
be  a  small  rhomboidal  face  (Fig.  3),  and  sometimes 
also  a  small  trapezoidal  face  (Fig.  2)  between  the 
latter  and  a  prism  face.  In  Fig.  2,  this  trapezoidal 
face  is  placed  on  the  right-hand  side  of  a  prism  face, 
and  the  crystal  is,  therefore,  a  right-handed  crystal. 
We  may  also  have  left-handed  crystals  of  quartz. 
Two  such  crystals,  a  right-handed  and  a  left-handed, 
may  be  exactly  alike,  except  that  one  is  the  mirror- 
reflection  of  the  other,  and  like  the  right  and  left 


QUARTZ  113 


hands  they  cannot  be  brought  into  coincident  po- 
sitions. 

A  specially  characteristic  feature  of  crystals  of 
quartz,  and  one  which  helps  in  the  recognition  of  the 
mineral,  is  that  the  prism  faces  are  always  more  or 
less  deeply  striated,  or  grooved,  horizontally;  that  is, 
they  are  marked  by  the  fine  lines  perpendicular  to 
the  edge  of  the  prism.  In  many  other  minerals  which 
crystallize  in  prismatic  forms — e.g.  beryl  and  topaz 
— the  prism  faces  are  more  usually  striated  parallel  to 
the  prism  edges. 

To  proceed  still  further  with  the  crystallography 
of  quartz — about  which,  indeed,  a  whole  volume 
might  be  written — it  may  be  remarked  that  not  only 
are  the  shapes  of  the  crystals  and  the  striations  char- 
acteristic and  peculiar  to  the  mineral,  but  also  the 
magnitude  of  the  solid  angles  between  the  faces  are 
equally  characteristic  and  constant  for  the  species.  It 
was  determined,  so  long  ago  as  1669,  by  Nicolaus 
Steno,  a  Danish  physician,  that  the  angle  between 
adjacent  prism  faces  is  always  exactly  120°,  and  that 
the  angle  between  a  prism  face  and  a  pyramid  face  is 
about  142°  (more  exactly  141°  47'  of  arc).  The 
angle  of  slope*  of  the  pyramid  faces  to  the  horizontal 
is  thus  very  nearly  52°,  which  curiously  is  the  same  as 
that  of  the  Pyramids  of  Egypt.  Again,  at  the  top 
of  the  crystal  the  angle  between  alternate  pyramid 
faces  is  94°  14',  or  nearly  a  right  angle;  so  that,  when 
three  alternate  faces  are  largely  developed  at  the  ex- 

*In  Plate  ix,  Fig.  I,  the  crystals  are  not  drawn  quite  correctly  in 
this  respect,  the  pyramid  being  rather  too  low. 


114        THE   WORLD'S    MINERALS 

pense  of  the  other  three,  the  crystal  may  assume  a 
cube-like  aspect. 

The  shapes  of  the  faces  themselves  are  likewise 
characteristic,  and  the  plane  angles  between  their 
edges  are  also  fixed  in  magnitude.  The  prism  faces 
are  rectangular  in  outline,  with  angles  of  90°,  and 
the  striations  on  them  are  parallel  to  one  pair  of 
edges.  The  pyramid  faces  when  equal  in  size  (as  in 
Fig.  2)  have  the  shape  of  acute  isosceles  triangles, 
with  angles  of  70°  at  the  base  and  40°  at  the  apex,  and 
with  the  sides  half  as  long  again  as  the  base.  It  will 
be  found  useful  to  bear  these  figures  in  mind.  For 
quartz  frequently  occurs  minutely  crystallized  in 
cavities  and  crevices,  forming  so-called  "drusy"  sur- 
faces; and  if  such  material  be  examined  with  a  mag- 
nifying-glass,  the  characteristic  shapes  of  the  prism 
and  pyramid  faces  can  usually  be  distinguished  when 
the  tiny  facets  catch  the  light. 

There  being  no  cleavage,  the  fracture  of  quartz  is 
typically  conchoidal,  and  on  the  rounded,  shell-like 
surfaces  of  fracture  the  luster  is  bright  and  vitreous 
in  character.  A  fractured  surface  of  quartz  presents 
exactly  the  appearance  of  broken  glass.  Amethyst 
shows  on  its  fractured  surface  a  very  peculiar  and 
characteristic  appearance;  it  is  marked  with  minute 
ripples  or  "thumb-marks,"  just  like  those  formed 
when  the  thumb  is  pressed  against  some  plastic  sub- 
stance. 

Quartz  when  perfectly  pure  is  quite  transparent 
and  colorless,  this  being  the  variety  known  as  rock- 
crystal.  The  specimen  represented  in  Plate  n,  Fig. 


OXIDES  (Quartz  group). 


Plate  11. 


5 

1,  Amethyst.    2,  Smoky-quartz.    3,  Quartz.    4,  Cat's-e5yei  :  ^  Rtofe-quarti, 


QUARTZ  115 


3,  is  from  the  Swiss  Alps.  Such  specimens  were  be- 
lieved by  the  ancients  to  be  ice,  frozen  so  hard  on  the 
highest  peaks  of  the  Alps  that  it  could  not  be  again 
thawed.  It  is,  indeed,  owing  to  this  belief  that  rock- 
crystal  received  its  name,  the  word  krystallos  meaning 
in  Greek  "clear  ice."  In  the  seventeenth  century 
this  name  came  to  be  extended  to  the  other  plane- 
faced  bodies  which  we  now  know  collectively  as 
crystals. 

The  distinction  between  other  varieties  of  crystal- 
lized quartz  depends  solely  on  differences  in  color. 
Clear  yellow  crystals  are  known  as  citrine,  occidental 
topaz,  or  false  topaz;  and  those  of  a  more  pronounced 
brown  color,  as  smoky-quartz  (Plate  n,  Fig.  2;  rep- 
resenting a  specimen  from  Switzerland)  or  cairn- 
gorm, after  the  locality  in  Scotland  where  such  crys- 
tals are  found.  Amethyst  (Fig.  i;  from  Brazil)  is 
violet,  or  purple,  crystallized  quartz.  Rose-quartz 
(Fig.  5;  from  Rabenstein,  in  Bavaria)  is  of  a  fine 
rose-red  color;  this  variety  is,  however,  found  only  as 
broken  masses,  without  crystal  faces. 

Another  variety  known  as  cat's-eye  (Plate  n,  Fig. 
4;  from  Ceylon)  owes  its  special  appearance  to  the 
enclosure  in  the  crystallized  quartz  of  large  numbers 
of  very  fine  fibers  of  asbestos  arranged  parallel  to 
each  other;  or  sometimes  the  asbestos  may  have  been 
dissolved  out  of  the  stone,  leaving  hollow  canals. 
Such  stones,  especially  when  cut  and  polished  with 
a  convex  surface  (as  represented  in  the  picture),  ex- 
hibit a  milky  band  of  reflected  light,  and,  as  the  stone 
is  moved  about,  this  band  travels  across  the  surface. 


116        THE    WORLD'S    MINERALS 

A  fancied  resemblance  to  the  eye  of  a  cat  gives  these 
stones  their  name  of  catVeye,  while  the  special  op- 
tical effect  is  known  as  chatoyancy.  Strictly,  how- 
ever, they  should  be  called  quartz  cat's-eye,  for  the 
same  effect  may  be  shown  by  other  minerals — for 
example,  chrysoberyl,  tourmaline,  and  gypsum — 
which  may  accidentally  possess  an  internal  fibrous 
structure.  Quartz  cat's-eyes  are  usually  pale  yellow- 
ish or  greenish  in  color,  but  those  from  the  Asbestos 
Mountains,  on  the  Orange  River,  in  South  Africa, 
display  a  magnificent  golden-yellow  sheen.  The 
latter  are  known  by  the  special  name  of  tiger-eye,  and 
often,  erroneously,  as  crocidolite  (compare  the  de- 
scription of  Plate  26,  Fig.  3). 

Coming  now  to  the  several  varieties  of  crypto- 
crystalline  or  compact  quartz,  these  consist  not  of 
single  crystals,  but  of  aggregates  of  vast  numbers 
of  minute  crystalline  individuals,  or  interlocking 
grains  so  closely  crowded  together  that  they  have 
had  no  opportunity  to  develop  crystal  faces  at  their 
boundaries.  The  exact  mode  of  aggregation  may  of 
course  be  varied,  giving  rise  to  a  corresponding  num- 
ber of  varieties  distinguished  by  special  names. 

Jasper  consists  of  such  an  aggregate,  the  minute 
grains  of  quartz  being  here  much  intermixed  with 
clayey  material  and  with  red  or  yellow  oxides  and 
hydroxides  of  iron.  These  impurities  give  rise  to  the 
different  colors  (red,  yellow,  brown,  green,  blue), 
which  may  be  present  singly  or  together  in  the  same 
piece  of  stone.  When  the  arrangement  of  colors  is 
in  parallel  bands,  we  have  the  variety  called  riband- 


OXIDES  (Quartz  group). 


Plate  12. 


1,  Agate.    2,  3,  Jasper.    4,  Hornstone. 


QUARTZ  117 


jasper.  In  the  red  jasper  shown  in  Plate  12,  Fig.  3, 
the  coloration  is  nearly  uniform,  while  in  the  ball- 
jasper  in  Fig.  2  (from  Freiberg,  in  Baden)  there  is 
a  concentric  banding  of  red  and  yellow.  Hornstone 
(Fig.  4)  is  of  much  the  same  character,  but  with 
much  less  admixed  impurity;  it  presents  a  certain 
amount  of  translucency,  with  dull  greyish  or  brown- 
ish colors,  so  that  it  somewhat  resembles  horn  in 
appearance.  Its  fracture  is  splintery. 

Chalcedony  is  a  rather  special  variety  of  crypto- 
crystalline  quartz,  in  which  the  structure,  as  seen  on 
a  fractured  surface,  is  minutely  fibrous.  On  the  sur- 
face it  presents  rounded  (mamillated,  botryoidal,  and 
stalactitic)  forms,  with  a  semi-transparent  appear- 
ance, and  a  luster  like  that  of  wax.  Here,  again,  we 
have  several  varieties  depending  on  differences  in 
color  and  the  arrangement  of  differently  colored 
portions.  The  color  of  common  chalcedony  is  white 
or  creamy.  When  it  is  bright  orange-red,  we  have 
the  well-known  carnelian;  and  when  brown,  the  sard. 
Pale  apple-green  chalcedony  is  known  as  chrysoprase, 
and  that  of  a  dark  leek-green  color  as  plasma.  An 
interesting  variety  of  a  dark-green  color,  marked 
with  bright  red  spots,  like  drops  of  blood,  is  called 
bloodstone,  or  heliotrope.  When  it  is  banded  and  of 
different  colors,  the  bands  may  be  more  or  less  con- 
centric, as  in  agate  (Plate  12,  Fig.  i),*  or  straight,  as 
in  onyx.  Again,  we  may  have  in  chalcedony  enclo- 

*In  this  figure  the  banding  in  the  outer  portion  is  not  so  clearly 
defined  as  may  usually  be  seen  in  an  actual  specimen.  On  the  specimens 
shown  in  Figs.  1-3  the  surfaces  have  been  ground  flat,  and  polished. 


118        THE   WORLD'S    MINERALS 

sures  of  coloring  matter,  bearing  in  their  form  some 
resemblance  to  moss  and  other  plant-like,  or  den- 
dritic, forms,  as  in  moss-agate  and  mocha-stone. 

As  to  the  practical  uses  to  which  quartz  is  applied, 
these  are  extremely  numerous.  In  the  form  of  sand- 
stone, it  is  much  used  for  building  and  paving.  Pure 
quartz-sand,  the  so-called  silver-sand,  is  extensively 
used  in  the  manufacture  of  glass,  and  also  for  abrasive 
purposes  (sand-paper)  and  for  scouring,  being  often 
mixed  with  soap.  Clear  rock-crystal  is  employed  for 
the  construction  of  prisms  and  lenses  for  optical 
purposes;  it  is  the  "Brazilian  pebble"  of  the  spec- 
tacle-makers, though  quartz  is  much  less  used  now 
than  formerly  for  spectacle-lenses.  Recently  fused 
quartz,  or  silica-glass,  has  been  much  used  for  the 
construction  of  various  pieces  of  apparatus  for  the 
chemical  laboratory. 

Several  varieties,  especially  those  which  are  clear 
or  of  a  fine  color,  are  extensively  used  in  jewelry  as 
semi-precious  stones.  Of  these,  amethyst  is  the  most 
valuable;  others  whose  names  are  familiar  are  car- 
nelian,  chrysoprase,  agate,  moss-agate,  etc.  Onyx  is 
engraved  as  cameos,  and  small  ornaments  of  various 
kinds  are  carved  in  agate.  Rock-crystal  was  carved 
by  the  ancients  to  form  valuable  crystal  vases  and 
bowls;  while  at  the  present  day  spheres  of  rock- 
crystal  (or,  perhaps,  often  of  glass)  find  a  ready  sale 
to  the  so-called  crystal-gazers.  Jasper,  being  found 
in  larger  masses,  is  used  as  polished  slabs  for  the 
decoration  of  both  the  inside  and  the  outside  of 
buildings. 


OPAL  119 


OPAL 

(Plate  10,  Figs.  4  and  5). — Opal  is  one  of  the  very 
few  minerals  that  exhibit  no  trace  of  crystalline  form. 
Not  even  under  the  microscope  can  any  indication  of 
an  internal  crystalline  structure  be  detected.  The 
mineral  is  amorphous,  like  glass;  it  is,  in  fact,  merely 
a  solidified  jelly,  and  has  been  formed  by  the  drying 
up  of  masses  of  gelatinous  hydrated  silica.  A  small 
but  variable  proportion  of  water  (3  to  10  per  cent.) 
is  still  retained  by  the  solid.  Chemically,  opal  has 
the  composition  of  quartz,  with  the  addition  of  water. 

Opal  usually  occurs  as  a  filling  in  cavities  and  fis- 
sures in  rocks  of  various  kinds,  and  it  is  also  found  in 
metalliferous  veins.  Since  it  usually  completely  fills 
the  cavity  in  which  it  is  found  (Plate  10,  Fig.  5),  it 
only  rarely  has  an  opportunity  of  developing  any 
external  boundaries  of  its  own;  and  these,  when 
present,  are  always  rounded  (botryoidal  or  stalac- 
titic)  surfaces.  The  fractured  surfaces  of  the  mineral 
are  curved  or  conchoidal,  like  those  of  broken  glass 

(Fig-  5)- 
In  its  external  appearance  opal  is  very  variable; 

and  to  gain  some  idea  of  its  general  character  the 
student  should  inspect  a  series  of  specimens  in  a  col- 
lection of  minerals.  It  may  be  perfectly  colorless 
and  transparent,  as  in  the  variety  known  as  hyalite, 
or  Muller's  glass;  but  more  often  it  is  semi-trans- 
parent to  opaque,  and  of  various  colors.  Frequently 
it  displays  a  kind  of  milky  appearance,  or  opal- 


120        THE    WORLD'S    MINERALS 

escence.  Further,  the  character  of  the  luster  may 
vary  from  glassy  to  waxy  (wax-opal)  or  pitchy 
(pitch-opal).  The  mineral  is  not  very  hard;  it  can 
be  scratched  by  quartz,  but  it  will  itself  scratch  glass. 
The  specific  gravity  (1.9-2.3)  is  appreciably  lower 
than  that  of  quartz. 

Several  varieties  of  opal  are  distinguished  by  spe- 
cial names,  the  more  important  being  common  opal 
(Plate  10,  Fig.  4)  and  precious  opal  (Fig.  5).  The 
precious  opal,  with  its  gorgeous  colors  flashing  and 
changing  with  every  movement  of  the  stone,  is  fa- 
miliar to  everybody.  These  magnificent  colors  are 
not,  however,  inherent  in  the  substance  of  the  opal 
itself;  for  if  a  precious  opal  be  held  up  between 
the  eye  and  the  window,  so  that  the  rays  of  light 
pass  through  the  stone,  only  a  pale  yellowish  or 
milky  color  is  to  be  seen.  The  play  of  rainbow  colors 
seen  by  reflected  light  is  entirely  an  optical  effect, 
and  is  due  to  the  interference  of  the  rays  of  light  at 
the  surfaces  of  fissures  or  internal  films.  Such  colors 
are,  of  course,  only  to  be  seen  in  white  light;  in  the 
yellow  or  red  light  of  a  photographer's  dark  room 
a  precious  opal  will  show  no  play  of  colors.  Certain 
specimens  exhibit  a  play  of  colors  only  when  they 
are  immersed  in  water  and  the  liquid  has  penetrated 
into  the  pores  of  the  stone. 

Localities  at  which  common  opal  is  found  are  very 
numerous  (the  specimen  represented  in  Plate  10,  Fig. 
4,  is  from  Hungary)  ;  but  precious  opal  is  of  much 
more  restricted  occurrence.  Before  the  discovery  of 
the  rich  opal-fields  of  Australia  (New  South  Wales 


OPAL— HAEMATITE  121 

and  Queensland)  practically  all  the  precious  opal 
used  in  jewelry  came  from  Hungary,  and  it  was  then 
much  more  valuable.  In  Hungary  the  mother-rock 
of  the  opal  is  a  volcanic  rock  known  as  trachyte;  in 
New  South  Wales  it  is  sandstone,  and  in  Queensland 
a  hard  siliceous  iron-stone  (Fig.  5). 

A  peculiar  variety  of  opal  is  the  siliceous  sinter, 
or  geyserite,  which  is  deposited  in  a  great  variety  of 
fantastic  forms,  and  as  an  encrustation  on  vegetable 
matter,  from  the  water  of  the  hot  springs  of  Iceland, 
the  Yellowstone  National  Park  in  the  United  States, 
and  in  New  Zealand.  Somewhat  similar  in  charac- 
ter is  the  powdery,  white  material  known  as  diato- 
mite,  or  infusorial  earth  (also  called  tripolite  and 
Kieselguhr),  which  consists  of  the  siliceous  skeletons 
of  vast  numbers  of  diatoms.  These  microscopic  or- 
ganisms inhabit  both  fresh  water  and  salt  water,  and 
their  remains  accumulate  on  the  floor  of  the  ocean 
and  of  lakes,  forming  sometimes  thick  deposits.  This 
material  is  largely  used  for  polishing;  it  is  highly 
absorbent,  and  when  mixed  with  nitro-glycerine 
forms  the  explosive  known  as  dynamite. 

HAEMATITE 

(Plate  13,  Figs.  1-4). — This  important  ore  of  iron 
is  an  oxide  (the  sesquioxide)  of  iron,  Fe2O3,  known 
to  chemists  as  ferric  oxide.  It  contains,  when  pure, 
70  per  cent,  of  iron.  When  crystallized,  it  is  black 
or  steel-grey,  with  a  brilliant  metallic  luster,  and  is 
then  known  as  iron-glance ,  or  specular  iron-ore;  but 


122        THE   WORLD'S    MINERALS 

when  massive  it  is  dull  red,  and  is  then  commonly 
called  red  iron-ore.  These  two  varieties  thus  differ 
considerably  in  their  external  appearance;  but  it  will 
be  found  that  when  scratched  with  a  knife  or  crushed 
they  yield  a  characteristic  dark  red  powder.  The 
resemblance  of  this  color  to  that  of  dried  blood  is  the 
reason  for  the  name  hamatite,  which  in  Greek  means 
"bloodstone." 

Crystals  of  haematite  belong  to  the  rhombohedral 
system,  but  they  vary  considerably  in  habit.  Usually, 
however,  the  crystals  are  more  or  less  tabular,  as  in 
Figs,  i  and  4,  and  sometimes  they  have  the  form  of 
quite  thin  plates  or  scales,  as  in  the  so-called  mica- 
ceous iron-ore. 

The  massive  varieties  often  exhibit  an  internal 
radiated  structure  (Figs.  2  and  3),  and  when  natural 
external  surfaces  are  present  these  are  rounded  or 
nodular  in  form — hence  the  name  kidney  iron-ore 
(Fig.  2).  Such  masses  may  be  quite  soft  and  earthy, 
being  then  of  a  brighter  red  color;  or  they  may  be 
harder  and  more  compact,  with  a  darker  color.  The 
fibrous  masses  are,  indeed,  sometimes  almost  as  hard 
and  black  as  the  crystals  themselves;  and  as  they  can 
be  split  up  along  the  fibers  into  thin  rods,  such  ma- 
terial is  know  as  pencil-ore. 

Large  deposits  of  crystallized  haematite  (specular 
iron-ore)  occur  in  the  island  of  Elba,  where  they 
have  been  extensively  worked  as  an  ore  ever  since 
the  time  of  the  Romans.  The  specimen  shown  in 
Plate  13,  Fig.  i,  is  from  these  iron-mines.  Kidney 
iron-ore  is  raised  in  large  quantities  from  the  several 


OXIDES. 


Plate  13. 


1—4,  Haematite. 


HEMATITE— MAGNETITE          123 

mines  in  north  Lancashire  and  west  Cumberland 
(Figs.  2  and  3),  where  the  ore  fills  large  cavities  in 
limestone.  In  these  mining  districts  the  red  color  of 
the  haematite  is  everywhere  in  evidence.  The  crystals 
represented  in  Fig.  4  are  not  from  a  mining  district; 
they  are  found  singly,  and  in  small  numbers,  attached 
to  the  walls  of  crevices  in  mica-schist  in  the  Binn 
valley  in  Switzerland.  These  crystals  contain  a  con- 
siderable amount  of  titanium  dioxide,  in  addition  to 
iron  oxide,  and  they  are  sometimes  regarded  as 
belonging  to  the  closely  allied  mineral  ilmenite. 
Haematite  is  also  found  in  some  volcanic  districts 
(Vesuvius  and  Madeira)  as  thin,  very  brilliant 
plates,  encrusting  the  surface  of  the  lava. 

The  chief  use  of  haematite  is,  of  course,  as  an  ore 
of  iron.  The  powdered  mineral  is  also  used  for  pol- 
ishing (jeweler's  rouge),  and  in  the  manufacture 
of  paints  and  red  pencils.  The  hard,  compact  pencil- 
ore  is  cut  and  polished  as  pin-stones  and  ring-stones 
for  use  in  jewelry;  it  takes  a  very  brilliant  polish, 
and  is  of  a  deep  black  color.  The  same  material, 
when  polished,  is  also  used  by  jewelers  and  book- 
binders for  burnishing  tools. 

MAGNETITE 

(Plate  14,  Fig.  i). — Magnetite,  or  magnetic  iron- 
ore,  is  another  important  ore  of  iron,  which  also  con- 
sists of  oxide  of  iron.  Here,  however,  it  is  the  oxide 
Fe3O4,  containing  rather  more  iron  (72.4  per  cent), 
and  correspondingly  less  oxygen,  than  haematite. 


124        THE    WORLD'S    MINERALS 

Chemically,  it  may  be  regarded  as  a  combination  of 
ferric  oxide  (Fe2O3)  and  ferrous  oxide  (FeO). 

In  its  strongly  magnetic  character,  magnetite  is 
unique  amongst  minerals,  no  other  even  approaching 
it  in  this  respect.  Fragments  of  the  mineral  are 
readily  attracted  by  a  magnet;  and  a  freely  suspended 
magnetic  needle  may  be  turned  completely  round  by 
holding  near  to  it  a  piece  of  the  mineral.  Further, 
certain  specimens  are  not  only  magnetic,  but  mag- 
netic with  polarity;  one  corner  or  end  of  the  speci- 
men which  attracts  one  pole  of  the  magnetic  needle 
will  repel  the  other  pole;  and  if  the  specimen  be 
freely  suspended  it  will,  like  a  magnetic  needle,  set 
itself  in  a  north  and  south  position  under  the  influ- 
ence of  the  earth's  magnetism.  Such  specimens  are 
the  well-known  lodestones,  or  natural  magnets. 

Crystals  of  magnetite  are  not  uncommon,  but  the 
mineral  is  more  usually  found  as  compact  or  granu- 
lar masses.  These  are  dull,  black,  and  heavy,  with 
no  very  characteristic  appearance;  but  their  identity 
with  magnetite  may  be  at  once  determined  simply 
by  holding  them  near  a  magnetic  needle.  The  streak, 
or  powder,  of  the  mineral  is  always  black. 

The  crystals  have  the  form  of  the  regular  octahe- 
dron, or  sometimes  of  the  rhombic-dodecahedron. 
In  the  picture  (Fig.  i)  are  shown  several  lustrous 
black  octahedra  on  the  surface  of  mica-schist.  This 
specimen  is  from  the  Binn  valley  in  Switzerland, 
where  crystals  of  magnetite  are  found  under  exactly 
the  same  conditions  as  crystals  of  haematite  (Plate 
13,  Fig.  4).  Excellent  crystals  of  magnetite  are  also 


MAGNETITE— SPINEL  125 

found  in  the  iron-mines  at  Traversella,  in  Piedmont, 
and  Nordmark,  in  Sweden. 

As  an  ore  of  iron,  magnetite  is  of  considerable  im- 
portance, most  of  the  Swedish  ores  being  of  this 
class,  and  it  is  also  extensively  mined  in  the  Urals 
and  in  New  York. 

SPINEL 

Magnetite  is  a  member  of  the  spinel  group  of 
minerals,  all  of  which  crystallize  in  the  cubic  system, 
usually  in  octahedral  forms;  and  further,  they  are 
analogous  in  chemical  constitution,  though  not  in 
actual  composition.  Taking  the  formula  of  magne- 
tite, Fe3(X  or  FeO.Fe2O3,  if  we  replace  the  ferrous 
oxide  by  magnesium  oxide  and  the  ferric  oxide  by 
aluminium  oxide,  we  arrive  at  the  formula  of  spinel, 
MgO.AloOs.  Similarly,  chromite,  or  chrome-iron- 
ore,  another  member  of  this  group,  consists  of  ferrous 
oxide  with  chromic  oxide,  FeO.Cr2O3;  while  an  ore 
of  zinc  known  as  gahnite  has  the  analogous  formula 
ZnO.Al2O3.  All  these,  and  several  other  minerals, 
belong  to  the  spinel  group. 

Spinel  itself  is  of  interest  in  being  one  of  the 
precious  stones  used  in  jewelry.  It  is  found  in  the 
gem-gravels  of  Ceylon  as  small,  red,  octahedral 
crystals;  and  when  cut  and  polished  it  presents  a 
striking  resemblance  to  the  true  ruby,  being,  in  fact, 
known  as  spinel-ruby,  or  balas-ruby.  It  may,  how- 
ever, be  readily  distinguished  from  the  true  ruby  by 
its  optical  characters,  specific  gravity,  and  hardness. 


126        THE    WORLD'S    MINERALS 


CORUNDUM 

(Plate  14,  Fig.  2). — The  oxide  of  the  light  and 
useful  metal  aluminium  is  known  to  the  chemist  as 
alumina  (A12O3),  to  the  mineralogist  as  corundum, 
to  the  jeweler  as  ruby  and  sapphire,  and  to  the  lapi- 
dary as  emery.  All  these  substances,  or  rather  vari- 
eties, of  the  mineral  corundum,  though  differing  so 
widely  in  their  external  and  general  appearance,  are 
identical  in  their  essential  characters — that  is,  chem- 
ical composition,  crystalline  structure,  hardness, 
specific  gravity,  and  other  physical  and  optical  char- 
acters. The  differences  they  exhibit  depend  solely  in 
the  presence  of  smaller  or  greater  amounts  of  ad- 
mixed coloring  matter  or  impurities.  A  convincing 
proof  of  this  is  that  the  same  stone,  even  when  faceted 
as  a  gem,  may  show  bands  of  different  colors. 

When  quite  pure,  crystallized  corundum  is  per- 
fectly colorless  and  transparent.  But  when  small 
amounts  of  various  inorganic  coloring  materials  are 
present  the  crystals  may  be  brilliantly  colored,  or, 
we  may  say,  dyed,  and  yet  retain  their  transparency. 
The  colors  of  such  transparent  stones  of  gem-quality, 
or  precious  corundum,  may  be  red,  orange,  yellow, 
green,  blue,  or  violet — all  the  colors  of  the  rainbow. 
These  differently  colored  stones  are  unfortunately 
designated  by  special  names  in  jewelry,  much  to  the 
confusion  of  the  uninitiated.  It  would,  indeed,  be 
just  as  reasonable  to  call  silk  ribbon  by  different 
names  according  to  the  color  it  has  been  dyed. 


CORUNDUM  127 


When  the  crystals  are  cloudy,  or  opaque,  due  to 
the  presence  of  fissures  and  the  enclosure  of  foreign 
materials,  the  mineral  is  known  as  common  corun- 
dum, as  distinct  from  precious  corundum.  When 
it  is  granular  and  mixed  with  a  considerable  amount 
of  foreign  matter,  especially  magnetite,  we  have  the 
variety  known  as  emery. 

These  two  more  abundant  forms  of  corundum  are 
of  considerable  importance  for  abrasive  purposes, 
being  used  for  the  making  of  corundum-wheels  and 
emery-cloth,  or  as  powder  for  grinding  precious 
stones  and  in  lapidaries'  work  generally.  This  ap- 
plication of  corundum  depends,  of  course,  on  its 
great  hardness  (No.  9  on  the  scale).  With  the  ex- 
ception of  diamond,  corundum  is  the  hardest  of  all 
minerals.  Recently,  however,  its  place  as  an  abrasive 
agent  has  been  taken  by  the  still  harder  artificial 
product  carborundum,  a  silicide  of  carbon. 

Crystals  of  corundum  belong  to  the  rhombohedral 
system.  The  crystal  represented  on  the  matrix  of 
crystalline  limestone  in  Plate  14,  Fig.  2,  consists  of 
a  hexagonal  prism  truncated  at  right  angles  by  the 
base,  and  with  small  triangular  faces  of  a  rhombo- 
hedron  on  three  of  its  corners.  A  more  usual  form 
for  crystals  of  sapphire  is,  however,  a  steeply  in- 
clined six-sided  pyramid. 

The  specific  gravity  (4.0)  of  corundum  is  con- 
siderably greater  than  that  of  metallic  aluminium 
(2.5),  and,  with  the  exception  of  zircon  and  some 
varieties  of  garnet,  it  is  the  heaviest  of  the  precious 
stones.  Why  the  combination  of  the  light  metal 


128         THE    WORLD'S    MINERALS 

aluminium  with  the  gas  oxygen  should  produce  a 
stone  of  such  density  is  difficult  to  understand. 

An  interesting  optical  effect  is  exhibited  by  the 
varieties  known  as  star-sapphire  and  star-ruby.  When 
cut  and  polished  with  a  convex  surface,  these  show 
a  six-rayed  star  of  reflected  light,  which  travels  over 
the  surface  as  the  stone  is  moved  about,  just  as  does 
the  single  band  in  the  catVeye,  mentioned  on  p.  115. 

Corundum  occurs  embedded  in  granite,  gneiss, 
serpentine,  or  crystalline  limestone.  It  is  not  a  widely 
distributed  mineral,  though  abundant  at  certain  lo- 
calities. It  is  mined  on  a  large  scale  for  abrasive 
purposes  in  Ontario  and  North  Carolina;  and  there 
have  long  been  important  emery-mines  in  Asia  Minor 
and  the  island  of  Naxos.  Rubies  of  the  best  quality 
are  those  found  in  a  crystalline  limestone,  or  marble, 
at  Mogok,  in  Upper  Burmah.  The  best  sapphires 
are  from  the  gem-gravels  of  Ceylon,  and  stones  of 
good  gem-quality  are  also  found  in  Australia  and  the 
United  States. 

Of  special  interest  are  the  variously  colored  cor- 
undums  of  gem-quality,  which  within  the  last  few 
years  have  been  made  artificially  in  large  quantities 
by  a  cheap  and  simple  process.  These  stones  possess 
all  the  characters  of  natural  crystallized  corundum, 
differing  from  it  only  in  their  mode  of  origin,  and 
when  cut  and  polished  they  furnish  handsome  gems. 


CASSITERITE  129 


CASSITERITE 

(Plate  14,  Fig.  3). — Cassiterite,  or  tin-stone,  is 
practically  the  only  ore  of  tin,  other  minerals  con- 
taining this  metal  being  of  quite  rare  occurrence. 
It  is  the  dioxide,  SnO2,  and  contains  78.6  per  cent, 
of  metallic  tin. 

It  is  a  very  heavy  mineral  (sp.  gr.  7),  usually  of  a 
dark-brown  color,  though  this  may  vary  from  almost 
white  to  black.  Most  frequently  it  is  met  with  as 
compact  masses,  or  as  grains  disseminated  through 
the  rock,  but  crystals  are  not  uncommon.  Sometimes 
it  has  a  fibrous  structure  resembling  that  of  wood,  be- 
ing then  known  as  'wood-tin. 

The  crystals  are  tetragonal,  and  are,  as  a  rule, 
rather  small.  Their  form  is  that  of  a  square  prism 
terminated  by  a  square  pyramid.  These  forms  are, 
however,  not  always  easily  made  out,  and  in  the  pic- 
ture (Fig.  3)  they  are  not  very  obvious.  Here,  how- 
ever, a  still  more  characteristic  feature  of  crystals  of 
cassiterite  is  represented;  in  the  lower  left-hand 
corner  of  the  picture  are  seen  two  twinned  crystals, 
with  their  very  characteristic  re-entrant  angles.  The 
faces  of  the  crystals  are  very  brilliant,  with  a  luster 
like  that  of  diamond. 

As  above  mentioned,  the  most  characteristic  feature 
of  the  mineral  is  its  heaviness,  but  the  only  certain 
way  of  determining  a  specimen  devoid  of  crystals  is 
to  test  for  the  presence  of  tin.  This  may  be  readily 
done  by  heating  the  powdered  mineral,  mixed  with 


130        THE    WORLD'S    MINERALS 

sodium  carbonate,  on  charcoal  in  the  reducing  flame 
of  the  blowpipe,  when  bright  beads  of  metallic  tin 
are  obtained. 

Cassiterite  is  not  a  mineral  of  wide  distribution, 
but  in  certain  districts  it  is  met  with  in  considerable 
abundance.  Veins  of  tin-ore  usually  occur  in  the  im- 
mediate neighborhood  of  granite,  as  in  the  long- 
known  tin-mining  district  of  Cornwall.  With  the 
weathering  and  breaking-down  of  the  tin-bearing 
rocks,  the  heavy,  indestructible  tin-stone  collects  in 
the  beds  of  rivers,  and  is  then  known  as  stream-tin. 
Alluvial  deposits  of  stream-tin  are  largely  worked 
in  the  Malay  Peninsula  and  in  Australia.  Another 
district  which  was  formerly  of  considerable  im- 
portance is  in  the  mountains  between  Saxony  and 
Bohemia;  the  specimen  represented  in  Plate  14  is 
from  Schlaggenwald,  Bohemia. 

ZIRCON 

(Plate  14,  Fig.  4). — This  mineral  is  a  double 
oxide  of  zirconium  and  silicon,  with  the  formula 
ZrO2.SiO2.  Writing  this  formula  in  the  form  ZrSiCh, 
the  composition  is  represented  as  a  silicate  of  zir- 
conium. Owing,  however,  to  the  remarkable  simi- 
larity of  the  crystalline  form  of  zircon  to  that  of 
cassiterite,  it  seems  more  natural  to  class  this  mineral 
with  the  oxides  rather  than  with  the  silicates. 

Zircon  is  found  only  as  crystals,  though  these  may 
sometimes  be  much  rounded  and  water-worn,  es- 
pecially when  found  in  gem-gravels.  The  crystals 


OXIDES. 


Plate  14. 


1,  Magnetite.    2,  Corundum.    3,  Cassiterite.   4,  Zircon.   5,  Pitchblende  or  Uraninite, 

6,  Limonite. 


ZIRCON  131 


are  tetragonal,  consisting  of  a  square  prism  capped 
by  a  square  pyramid.  In  the  picture  (Fig.  4)  we  see 
two  such  crystals  partly  embedded  in  felspar  from 
the  Ilmen  Mountains,  in  the  southern  Urals;  one 
crystal,  seen  in  side  view,  shows  the  edge  of  the  large 
square  prism  truncated  by  narrow  planes  (only  one 
is  visible  in  the  picture)  of  a  second  square  prism; 
the  other  crystal,  seen  in  end  view,  shows  only  the 
four  faces  of  the  square  pyramid. 

The  color  of  zircon  is  somewhat  variable;  brown, 
as  shown  in  the  picture,  is  the  fmost  frequent  color 
of  the  crystals.  Reddish-brown  stones  of  gem-quality 
are  known  as  hyacinth,  and  green  and  yellow  stones 
are  sometimes  called  jargoon.  Owing  to  these  bright 
colors  and  to  its  high  degree  of  brilliancy  and  hard- 
ness, zircon,  when  cut  and  polished,  makes  a  very 
effective  gem-stone;  and  when  colorless  it  may  even 
be  mistaken  for  diamond.  Stones  of  a  reddish-brown 
color  may  be  completely  decolorized  by  heat,  and  are 
sometimes  sold  as  diamonds. 

A  very  remarkable  feature  presented  by  zircon  is 
the  wide  range  of  its  specific  gravity,  which  in  dif- 
ferent specimens  may  vary  from  4.0  to  4.7.  Further, 
certain  stones  when  heated  change,  not  only  in  color, 
but  also  in  specific  gravity. 

Zircon  is  an  almost  invariable  constituent  of  gran- 
ite and  certain  other  rocks  of  igneous  origin ;  but,  as 
a  rule,  the  crystals  are  extremely  minute,  and  present 
in  only  relatively  small  amount.  When  such  rocks 
are  broken  down  by  weathering,  the  zircon,  being 
indestructible,  is  carried  away  by  running  water  with 


132         THE    WORLD'S    MINERALS 

the  debris.  For  this  reason  microscopic  crystals  of 
zircon  may  be  detected  in  almost  all  sands,  gravels, 
and  sandstones.  The  larger  pebbles  and  water-worn 
crystals  of  zircon  so  abundant  in  the  gem-gravels  of 
Ceylon  have  in  the  same  way  been  derived  from  the 
granitic  rocks  of  the  island. 

PITCHBLENDE 

(Plate  14,  Fig.  5). — Pitchblende  is  an  ugly  black 
mineral,  which  in  color,  luster,  and  fracture  bears 
some  resemblance  to  pitch.  When,  however,  a  speci- 
men is  handled,  it  will  at  once  be  noticed  that  it  is 
far  heavier  than  pitch — far  heavier,  indeed,  than 
most  other  materials.  Its  specific  gravity  ranges  from 
8.0  to  9.7,  the  former  value  being  greater  than  the 
specific  gravity  of  iron,  and  the  latter  greater  than 
that  of  copper. 

In  spite  of  its  unattractive  appearance,  pitchblende 
has  within  recent  years  come  to  be  a  mineral  of  ex- 
traordinary interest  by  reason  of  its  remarkable  radio- 
active properties.  The  invisible  rays  continually 
being  emitted  by  a  specimen  of  pitchblende  are  ca- 
pable of  acting  on  a  photographic  plate;  so  that  it  is 
possible  with  its  aid  to  photograph  the  outlines  of 
objects  in  the  dark.  The  rays  may  also  cause  certain 
substances  (especially  artificially  prepared  hexagonal 
zinc  sulphide)  to  become  luminous,  or  to  phosphor- 
esce, in  the  dark.  Again,  they  possess  the  power  of 
making  the  air  a  conductor  of  electricity;  so  that 
if  a  piece  of  pitchblende  be  placed  near  a  charged 


PITCHBLENDE— LIMONITE         133 

gold-leaf  electroscope,  the  charge  of  electricity  rapid- 
ly escapes,  and  the  gold-leaves  fall  together. 

These  remarkable  properties  were  first  discovered 
in  uranium  compounds  by  the  late  Professor  Henri 
Becquerel  in  1896;  and  the  observation  that  they  are 
strongly  marked  in  pitchblende  led  Madame  Curie 
to  the  isolation  of  the  particular  elements — radium 
and  polonium — to  which  these  special  effects  are  due. 
The  amount  of  radium  contained  in  pitchblende — 
the  richest  ore  of  radium — is,  however,  extremely 
minute,  not  more  than  one  part  in  five  million.  Sev- 
eral tons  of  the  mineral  have  to  be  treated  by  a  long 
series  of  complex  chemical  operations  to  obtain  even 
a  very  small  amount  of  a  radium  compound. 

Chemically,  pitchblende  is  an  oxide  of  uranium, 
and  on  this  account  it  is  known  to  mineralogists  as 
uraninite.  Oxides  of  some  other  rare  metals  are  often 
also  present  in  variable  amount. 

The  most  productive  mines  of  pitchblende  are 
those  at  Joachimsthal,  in  Bohemia,  where  it  occurs 
in  metalliferous  veins,  together  with  ores  of  silver 
and  cobalt.  It  is  also  found  in  some  of  the  Cornish 
mines.  At  these  places  it  is  always  massive;  but  in 
the  felspar  quarries  in  the  south  of  Norway  it  is 
found  as  small  cubic  crystals  (known  by  the  special 
names  broggerite  and  cleveite)  embedded  in  the 
felspar. 

LIMONITE 

(Plate  14,  Fig.  6). — On  account  of  its  usual  rusty- 
brown  color  this  important  ore  of  iron  is  often  known 


134         THE    WORLD'S    MINERALS 

as  brown  iron-ore.  Certain  masses  when  presenting  a 
rounded  (nodular  or  stalactitic)  surface  may,  how- 
ever, be  black  and  shining;  but  when  such  masses 
are  broken  open,  or  scratched  with  a  knife,  the  char- 
acteristic brown  color  is  at  once  rendered  evident. 
These  more  compact  masses  usually  exhibit  an  in- 
ternal fibrous  structure,  with  a  banded  arrangement 
perpendicular  to  the  fibers  (Fig.  6).  Crystals  of 
limonite  are  quite  unknown.  Earthy  or  powdery 
masses  are  the  most  frequent. 

Limonite  is  a  hydrated  oxide  of  iron,  with  the 
chemical  formula  2Fe2O33H2O — that  is,  the  same  as 
haematite,  with  the  addition  of  water.  It  is  the  final 
product  of  weathering  of  all  iron-bearing  minerals. 

As  a  yellow,  powdery,  or  slimy  substance,  it  is  de- 
posited by  the  waters  of  chalybeate  springs;  and  ma- 
terial of  a  similar  nature  is  formed  in  marshes  and 
on  the  beds  of  certain  lakes.  The  latter  constitutes 
the  Swedish  lake-ore,  or  bog-iron-ore;  and  when  it 
has  been  all  collected  from  a  particular  spot  a  fresh 
deposit  is  ready  for  working  after  a  lapse  of  a  num- 
ber of  years.  Limonite  is  the  most  important  iron- 
ore  in  Germany,  and  large  quantities  are  also  mined 
in  the  north  of  Spain.  Besides  being  used  as  an  ore 
of  iron,  the  purer,  powdery  variety,  or  yellow-ochre, 
is  used  as  a  pigment 

MANGANESE  OXIDES 

(Plate  15). — The  three  species  manganite,  pyrolu- 
site,  and  psilomelane,  represented  on  Plate  15,  may 


OXIDES. 


Plate  15 


ii*S 

m, 


•F 


1,  Manganite.    2,  3,  Pyrolusite.    4,  Psilomelane. 


MANGANESE    OXIDES  135 

conveniently  be  treated  together,  for  they  closely  re- 
semble one  another  in  appearance,  and  are,  as  a  rule, 
not  readily  to  be  distinguished;  further,  they  occur 
together,  and  are  all  mined  and  put  to  the  same  uses. 

In  color  they  are  all  black,  usually  with  a  semi- 
metallic  luster;  their  streak,  or  powder,  is  also  black, 
and  some  varieties  are  so  soft  that  they  soil  the  fingers 
when  touched.  The  black  streak  is,  indeed,  the  most 
characteristic  and  constant  feature  of  these  minerals; 
if  we  have  a  soft,  black  mineral  which  gives  a  black 
streak  we  may  be  pretty  certain  that  it  is  one  of  these 
oxides  of  manganese. 

The  only  species  which  occurs  distinctly  crystal- 
lized is  manganlte  (Fig.  i ) .  The  crystals  are  usually 
prismatic  in  form,  and  at  a  few  localities  very  fine 
specimens  are  to  be  found,  but  elsewhere  they  are 
rare.  The  picture  shows  bundles  of  prismatic  crystals 
on  barytes,  and  represents  a  specimen  from  Ilfeld,  in 
the  Harz  Mountains,  Germany.  This  species  is  the 
hydrated  oxide,  Mn2O3.H2O. 

Another  species,  pyrolusite  (Figs.  2  and  3),  is  per- 
haps the  most  abundant  of  these  oxides  of  manganese. 
It  usually  forms  radiating  aggregates  of  platy  or 
needle-like  crystals,  as  represented  in  the  pictures. 
In  chemical  composition  it  approximates  to  the  di- 
oxide, MnO2. 

A  third  species,  psilomelane  (Fig.  4),  is  never 
found  crystallized,  but  usually  as  nodular  or  stalac- 
titic  masses  with  rounded  surfaces.  The  stalactites 
have  sometimes  an  internal  radiated  structure,  as 
shown  in  the  picture  (where  they  have  been  broken 


136        THE    WORLD'S    MINERALS 

across;  three  of  the  natural  rounded  ends  are  also 
shown).  The  surface  of  these  masses  is  often  very 
smooth  and  shining,  and  the  material  is  then  quite 
hard.  In  the  variety  known  as  wad,  the  material  is, 
however,  quite  soft,  and  sometimes  so  porous  that  it 
floats  on  water.  Psilomelane  and  wad  are  very  vari- 
able in  chemical  composition;  though  consisting 
largely  of  manganese  dioxide,  they  also  contain 
oxides  of  barium,  potassium,  cobalt,  etc. 

These  black  manganese  minerals  owe  their  origin 
to  the  alteration  by  weathering  of  other  minerals 
containing  manganese,  and  in  some  places  they  form 
large  deposits.  Such  deposits  are  mined  principally 
in  Brazil,  Russia,  and  India.  These  minerals  have 
long  been  used  for  the  manufacture  of  chlorine  and 
bleaching  powder  and  for  decolorizing  glass.  More 
recently,  large  quantities  have  been  used  for  prepar- 
ing the  spiegeleisen  and  ferro-manganese  employed 
in  the  manufacture  of  iron. 

CUPRITE 

The  only  remaining  oxide  which  need  be  men- 
tioned in  this  place  is  the  suboxide  of  copper  (or 
cuprous  oxide)  Cu2O.  It  is  sometimes  found  beauti- 
fully crystallized  in  cubes  or  octahedra,  and  the 
crystals,  being  very  brilliant,  transparent,  and  of  a 
rich  ruby-red  color,  are  very  attractive  in  appearance. 
On  this  account  the  mineral  is  also  known  as  ruby- 
copper,  or  red  copper-ore.  In  a  peculiar  variety 
known  as  chalcotrichite  (or  hair-copper),  the  cubes 


CUPRITE  137 


are  enormously  elongated  in  the  direction  of  one  of 
their  edges,  so  resembling  fine  hairs;  and  when,  as 
is  usually  the  case,  large  numbers  of  these  hair-like 
crystals  are  closely  aggregated  or  matted  together, 
the  material  has  the  appearance  of  crimson  plush. 
Another  variety  is  earthy  in  character  and  inter- 
mixed with  limonite,  and  from  its  appearance  it  is 
known  as  tile-ore.  In  some  copper-mining  districts 
cuprite  is  of  importance  as  an  ore. 


CHAPTER  IX 

THE  CARBONATES 

FROM  the  oxides  considered  in  the  last  chapter,  we 
pass  to  the  very  large  series  of  oxygen-salts,  and  the 
first  group  of  this  series  is  that  of  the  carbonates. 
These  consist  of  carbon  and  oxygen  in  combination 
with  one  or  other  of  the  metals;  that  is,  they  are 
salts  of  carbonic  acid  (H2CO3),  which  acid  in  its 
gaseous  form  is  the  well-known  carbon  dioxide 
(CO2),  produced  when  all  organic  substances  are 
burnt  in  the  air.  In  addition  to  these  elements,  some 
minerals  of  this  group  may  contain  the  elements  of 
water,  being  hydrated  carbonates;  while  others  may 
contain  an  excess  of  the  metal  in  the  form  of  hydrox- 
ide, these  being  hydrated  basic  carbonates. 

When  a  carbonate  is  heated  to  redness,  the  carbon 
dioxide  it  contains  is  expelled,  leaving  the  metallic 
oxide;  this  takes  place  when  limestone  (CaCOs)  is 
burnt  to  give  quicklime  (CaO).  Or  the  carbon  di- 
oxide may  be  expelled  simply  by  placing  the  car- 
bonate in  acid,  when  the  gas  that  is  liberated  bubbles 
through  the  liquid.  This  property  that  carbonates 
have  of  effervescing  in  contact  with  acid  affords  a 
simple  test  by  which  they  can  always  readily  be  de- 
tected. It  is  only  necessary  to  place  a  drop  of  acid 
(dilute  hydrochloric  acid  is  the  best)  on  a  mineral 

138 


CARBONATES 


Plate  16. 


1—3,  Calcite. 


CALCITE  139 


to  decide  whether  or  not  it  is  a  carbonate.  In  certain 
cases,  however,  it  may  be  necessary  to  place  the 
powdered  mineral  in  warm  acid  before  the  effer- 
vescence is  produced. 

Most  carbonates,  when  in  a  pure  state,  are  color- 
less and  transparent.  They  are  all  fairly  soft,  and 
can  be  scratched  with  a  knife.  Most  of  them  fall  into 
well-defined  isomorphous  groups. 

CALCITE 

(Plate  1 6,  Figs.  1-3). — Next  to  quartz,  calcite  or 
calc-spar  is  the  most  abundant  of  minerals,  and  it  also 
appears  in  a  great  variety  of  forms  or  guises.  The 
common  and  plentiful  rocks  of  the  limestone  class 
(chalk,  oolite,  marble)  consist  entirely,  or  almost  en- 
tirely, of  compact  calcite.  These  rocks  have  in  most 
cases  been  formed  by  the  accumulation  of  the  cal- 
careous remains  of  various  marine  organisms,  such 
as  foraminifera,  corals,  molluscs,  etc.,  which,  like 
calcite  itself,  are  composed  chemically  of  calcium 
carbonate  (CaCOs). 

Calcite  is  remarkable  for  the  frequency  with  which 
it  is  found  in  a  well-crystallized  condition;  indeed, 
calcite  crystals  are  the  commonest  of  all  kinds  of 
crystals.  Further,  they  display  a  greater  variety  of 
forms  than  is  met  with  in  any  other  mineral.  We 
may  have  plate-like  crystals,  which  are  sometimes  as 
thin  as  a  sheet  of  paper;  or  the  crystals  may  be  pris- 
matic in  habit,  with  either  a  hexagonal  or  a  triangular 
cross-section,  and  they  may  sometimes  be  long  and  as 


140        THE    WORLD'S    MINERALS 

slender  as  needles.  Or,  again,  we  may  have  rhombo- 
hedra,  either  flat  or  steep,  or  various  scalenohedral 
forms.  Calcite  of  the  several  forms  just  enumerated 
is  often  known  by  several  trivial  names,  such  as 
paper-spar,  cannon-spar,  rhomb-spar,  nail-head-spar, 
and  dog-tooth-spar.  Combinations  of  these  simple 
forms  give  rise  to  an  almost  endless  variety  of  forms 
for  crystals  of  calcite.  In  Plate  16,  Fig.  3,  the  form 
of  the  crystals  is  that  of  the  primitive  rhombohedron ; 
in  Fig.  2,  it  is  another  more  acute  rhombohedron; 
and  in  Fig.  i,  a  combination  of  a  hexagonal  prism,  a 
scalenohedron,  and  an  obtuse  or  flat  rhombohedron. 
Although  differing  so  widely  in  form  and  appear- 
ance, all  these  crystals  belong  to  one  common  type. 
They  are  each  symmetrical  with  respect  to  three 
planes  of  symmetry,  with  a  three-fold  arrangement 
about  the  vertical  axis ;  and  the  angles  between  sim- 
ilar faces  are  always  the  same. 

A  very  important  property  of  crystals  of  calcite  is 
that  of  splitting,  or  cleaving,  with  great  ease  in  three 
directions  parallel  to  the  faces  of  the  primitive  rhom- 
bohedron. Any  crystal  of  calcite,  whatever  be  its 
external  form,  may  be  broken  up  by  a  hammer  into 
a  number  of  small  rhombohedra,  with  smooth  and 
bright  faces.  This  cleavage  rhombohedron  is  of  the 
same  form  as  the  primitive  rhombohedron  (Fig.  3), 
with  identical  angles  (105°  5',  or  74°  55')  between 
adjacent  faces.  The  plane  angles  on  the  rhomb- 
shaped  faces  are  102°  and  78°. 

This  perfect  cleavage  of  calcite  is  of  the  first  im- 
portance as  an  aid  to  the  recognition  of  the  mineral. 


CALCITE  141 


Other  characters  of  importance  in  this  direction  are 
the  low  hardness  (No.  3  on  the  scale)  and  low 
specific  gravity  (2.72)  ;  the  mineral  can  be  readily 
scratched  with  a  knife.  Another  test  that  may  be 
readily  applied  is  given  by  the  fact  that  the  mineral 
effervesces  freely  with  cold,  dilute  acid.  A  minute 
fragment  of  calcite,  when  placed  in  a  drop  of  hy- 
drochloric acid  on  a  microscope  slide,  quickly  dis- 
solves with  effervescence;  on  adding  a  drop  of  sul- 
phuric acid  to  this  solution,  and  allowing  the  liquid 
to  evaporate,  star-like  groups  of  crystals  of  gypsum 
are  formed,  and  these  present  a  very  characteristic 
appearance  under  the  microscope. 

Crystals  of  calcite  vary  not  only  in  their  geometric- 
al form,  but  they  may  also  vary  considerably  in  their 
degree  of  transparency  and  color.  Most  frequently 
they  are  of  a  pale  yellowish  or  reddish  color,  owing 
to  the  enclosure  of  hydrated  oxides  of  iron,  and  are 
only  semi-transparent.  When  the  material  is  quite 
pure  it  is  perfectly  colorless  and  transparent.  The 
purest  crystallized  material  of  the  best  quality  comes 
from  one  spot  in  Iceland,  and  is  known  as  Iceland- 
spar.  This  is  much  used  for  making  optical  prisms 
for  polarizing  apparatus. 

The  use  just  mentioned  depends  on  a  very  peculiar 
property  possessed  by  crystals  of  calcite — namely, 
that  of  double  refraction,  a  property  which  is  pos- 
sessed by  many  other  minerals,  though  rarely  to  such 
a  high  degree.  If  a  cleavage-rhomb  of  Iceland-spar 
be  placed  over  an  object  (such  as  a  dot  or  a  cross 
drawn  on  a  sheet  of  paper) ,  the  object  will  be  shown 


142        THE    WORLD'S    MINERALS 

double  through  the  spar,  there  being  seen  two  dots 
or  two  crosses.  A  ray  of  light  on  entering  a  crystal 
of  calcite  is  split  into  two  rays,  which  travel  along 
different  paths  through  the  crystal.  For  this  reason 
Iceland-spar  is  also  known  as  doubly-refracting  spar. 

Beautiful  crystallized  specimens  of  calcite  are  to 
be  found  at  very  many  places.  One  of  the  best-known 
localities  is  in  the  west  Cumberland  iron-mining  dis- 
trict in  the  neighborhood  of  Egremont.  Here  large 
fissures  or  cavities  in  the  haematite  iron-ore  and  in  the 
adjacent  limestone  are  completely  lined  with  num- 
berless sparkling  crystals,  which  are  often  quite 
transparent  and  tipped  with  delicate  shades  of  red. 
Enormous  crystals  of  water-clear  calcite  have  been 
found  in  cavities  in  lava  in  Iceland;  and  magnificent 
crystals  of  a  rich  wine-yellow  or  wine-red  color  are 
met  with  in  the  lead  and  zinc  mines  near  Joplin,  in 
Missouri.  Many  other  well-known  localities  might 
easily  be  mentioned.  The  specimen  represented  in 
Plate  1 6,  Fig.  i,  is  from  Egremont,  in  Cumberland, 
the  crystals  here  shown  being  on  a  matrix  of  iron-ore. 
The  specimen  in  Fig.  2  is  from  Fontainebleau,  in 
France;  the  closely  aggregated  groups  of  rhombo- 
hedral  crystals  from  this  locality  are  remarkable  in 
enclosing  a  large  amount  of  sand. 

The  well-known  stalactites,  found  hanging  from 
the  roofs  of  limestone  caves,  consist  of  calcite.  These 
have  been  formed  by  the  deposition  of  calcium  car- 
bonate from  the  water  which  drips  from  the  roof  of 
the  caves. 

Calcite  may  be  taken  as  the  type  of  a  group  of 


CALCITE— CALAMINE  143 

minerals  known  as  the  rhombohedral  carbonates. 
These  all  crystallize  in  rhombohedra,  which  differ 
only  slightly  from  the  primitive  rhombohedron  of 
calcite,  there  being  a  difference  of  only  a  few  degrees 
in  the  angles  between  the  faces.  They  also  all  pos- 
sess the  same  perfect  cleavages  parallel  to  the  faces 
of  this  rhombohedron.  The  several  members  of  the 
group,  though  consisting  of  carbonates  of  different 
metals,  are  yet  analogous  in  their  chemical  consti- 
tution, for  the  metals  they  contain  are  closely  related. 
They  have  the  general  formula  R"CO3,  where  R" 
may  stand  for  calcium,  magnesium,  iron,  zinc,  or 
manganese.  We  have  here  an  excellent  example  of 
an  isomorphous  group  of  minerals. 

Three  of  the  minerals  of  this  group,  which  are 
represented  in  Plate  17,  Figs.i-3,  will  each  be 
specially  described  below.  Others,  which  we  may 
mention  by  name  in  this  place,  are  magnesite,  or 
magnesium  carbonate  (MgCOs),  and  dolomite 
(CaCOs.MgCOs),  in  which  both  calcium  and 
magnesium  are  present,  this  species  being  midway 
between  calcite  and  magnesite. 

CALAMINE 

(Plate  17,  Fig.  i). — This  is  the  carbonate  of  zinc 
(ZnCOs)  belonging  to  the  group  of  rhombohedral 
carbonates.  Distinctly  developed  rhombohedral  crys- 
tals are,  however,  quite  rare,  and  are  always  small. 
More  usually  the  mineral  forms  masses  with  con- 
centric shelly  layers  and  a  nodular  surface,  or  it  is 


144        THE    WORLD'S    MINERALS 

frequently  earthy  or  compact.  In  color  it  is  often 
creamy  or  yellowish,  though  sometimes  bluish  or 
greenish  (Fig.  i).  It  is  a  mineral  presenting  no 
very  distinctive  external  features,  and  it  is  frequently 
mistaken  for  other  minerals.  The  only  safe  way  of 
determining  it  is  to  test  for  the  presence  of  carbonate 
and  of  zinc,  the  latter  by  the  method  already  men- 
tioned under  zinc-blende. 

Calamine  occurs  in  metalliferous  veins,  together 
with  zinc-blende,  and  it  frequently  forms  extensive 
bedded  deposits  in  limestones.  It  contains  64.8  per 
cent,  of  zinc,  and  is  an  important  ore  of  this  metal. 

CHALYBITE 

(Plate  17,  Fig.  2). — The  rhombohedral  carbonate 
of  iron  (FeCOs),  known  as  chalybite  or  spathic  iron- 
ore,  or  sometimes  as  siderite,  is  often  found  as  small, 
well-developed  crystals  of  various  forms,  the  rhombo- 
hedron  being  the  most  frequent.  Brown  crystals  of 
this  kind  are  shown,  in  association  with  galena,  in 
Plate  5,  Figs.  3  and  4;  and  a  brown  encrustation  of 
chalybite,  on  fluor-spar,  is  shown  in  Plate  9,  Fig.  i. 
More  often  the  mineral  occurs  as  granular  masses 
showing  numerous  bright  cleavages  on  the  fractured 
surfaces;  sometimes  such  masses  may  be  very  coarse- 
ly crystalline,  with  large  cleavage  surfaces.  Fre- 
quently the  material  is  compact,  with  a  more  or  less 
pronounced  radiated  structure  (Plate  17,  Fig.  2). 
It  is  then  often  mixed  with  various  impurities,  such 
as  clay  and  carbonaceous  matter,  as  in  the  varieties 


CARBONATES. 


Plate  17. 


1,  Calamine.    2,  Siderite.    3,  Rhodochrosite.    4,  Corusske.  ** 


CHALYBITE— RHODOCHROSITE    145 

known  as  clay  iron-stone  and  black  band  iron-stone, 
which  are  of  great  importance  as  ores  of  iron. 

The  crystallized  varieties  of  chalybite  are  more 
usually  found  in  metalliferous  veins — for  instance,  in 
Cornwall  and  in  the  Harz  Mountains;  while  the 
granular  and  compact  varieties  form  beds  in  sedi- 
mentary rocks.  In  the  coal-measures  of  England 
and  South  Wales  we  have  the  abundantly -occurring 
clay  iron-stone  and  black  band  iron-stone.  The  im- 
portant iron-ores  of  the  Cleveland  district  of  York- 
shire consist  of  oolitic  chalybite,  forming  beds  in  the 
Lias  formation.  When  chemically  pure,  chalybite 
contains  48.2  per  cent,  of  iron,  but  usually,  owing  to 
the  presence  of  admixed  impurities,  the  ore  does  not 
contain  so  high  a  percentage  as  this. 

RHODOCHROSITE 

(Plate  17,  Fig.  3). — The  carbonate  of  manganese 
(MnCOa)  known  as  rhodochrosite,  or  manganese- 
spar,  is  very  different  in  its  appearance  from  the 
manganese  minerals  which  we  have  previously  con- 
sidered (p.  134).  Instead  of  being  black  and  dirty, 
it  is  of  a  beautiful  rose-red  color,  and  it  is  found  as 
clean-looking  rhombohedral  crystals  (Fig.  3).  These 
crystals  possess  the  perfect  rhombohedral  cleavage 
characteristic  of  this  group  of  carbonates,  and  con- 
sequently they  sometimes  exhibit  a  more  or  less 
marked  pearly  appearance  on  their  surfaces.  More 
frequently,  however,  the  faces  of  crystals  are  curved, 
sometimes  so  pronouncedly  that  the  crystals  some- 


146        THE   WORLD'S    MINERALS 

what  resemble  a  saddle  in  form.  The  mineral  also 
occurs  as  granular  masses,  with  a  globular  surface, 
being  then  sometimes  known  as  raspberry-spar,  as 
suggested  by  its  appearance. 

Crystallized  rhodochrosite  is  found  in  cavities  in 
limonite  at  Horhausen,  in  Rhenish  Prussia,  and  in 
metalliferous  veins  at  Kapnik,  in  Hungary.  The 
best  crystals  are  found  with  iron-pyrites  at  various 
places  in  Colorado  (Fig.  3).  The  massive  mineral 
forms  extensive  beds  in  limestone  in  the  Pyrenees, 
where  it  is  mined  as  an  ore  of  manganese.  This 
ore  cannot,  however,  be  used  for  the  same  purposes 
(bleaching,  etc.)  as  the  black  ores  of  manganese,  since 
it  contains  no  dioxide;  it  is  used  only  in  the  manu- 
facture of  iron. 

ARAGONITE 

(Plate  1 8,  Figs.  1-3). — In  chemical  composition 
aragonite  is  the  same  as  calcite,  being  calcium  car- 
bonate (CaCOs).  Nevertheless,  we  have  here  two 
quite  distinct  minerals,  which  differ  not  only  in  their 
crystalline  form,  but  also  in  all  their  physical  char- 
acters. We  have,  in  fact,  the  same  chemical  com- 
pound crystallizing  in  two  totally  distinct  modifica- 
tions, just  as  carbon  crystallizes  either  as  diamond 
or  as  graphite  (see  p.  63),  and  as  sulphur  forms 
either  orthorhombic  or  monoclinic  crystals  (p.  66). 
In  other  words,  calcite  and  aragonite  are  dimorphous 
forms  of  calcium  carbonate. 

In  its  general  appearance  aragonite  is,  however, 
not  unlike  calcite,  the  material  being  usually  color- 


ARAGONITE  147 


less,  white,  or  pale-colored,  either  transparent  or 
translucent,  and  with  a  glassy  luster.  And,  indeed, 
when  the  material  is  very  minutely  crystallized  it  is 
not  always  quite  easy  to  distinguish  between  calcite 
and  aragonite. 

Distinctly  formed  crystals  of  aragonite  are  com- 
paratively rare;  their  system  of  crystallization  is 
orthorhombic,  and  they  are  usually  prismatic  in  their 
development.  The  orthorhombic  form  is,  however, 
very  often  obscured  by  twinning,  and  frequently  three 
crystals  are  twinned  together  to  produce  a  form 
closely  resembling  a  hexagonal  prism,  as  shown  in 
Plate  1 8,  Fig.  3.  Twinned  crystals  of  this  kind  are 
found  in  Aragon,  Spain — hence  the  name  aragonite 
for  this  species;  in  the  sulphur-mines  near  Girgenti, 
in  Sicily;  at  Herrengrund,  in  Hungary  (Fig.  3)  ;  and 
in  the  copper-mines  at  Corocoro,  in  Bolivia,  where 
the  crystals  are  often  replaced  by  native  copper,  with 
their  original  form  still  preserved. 

More  frequently,  crystals  of  aragonite  have  the 
form  of  very  steep  spires,  spikes,  or  fine  needles, 
which  are  usually  aggregated  in  divergent  groups 
or  in  delicate  feathery  forms.  This  acicular,  or 
needle-like,  habit  of  the  crystals  is  very  character- 
istic of  aragonite;  but  it  is  not  correct  to  suppose  (as 
is  frequently  done)  that  all  needle-like  crystals  of 
calcium  carbonate  are  aragonite,  for  we  may  also 
have  acicular  crystals  of  calcite. 

We  have  already  seen  that  when  a  crystal  of  calcite 
is  broken,  the  surfaces  of  fracture  are  quite  plane  and 
smooth,  since  the  mineral  possesses  perfect  cleavages 


148         THE    WORLD'S    MINERALS 

parallel  to  the  faces  of  the  primary  rhombohedron. 
In  aragonite,  on  the  other  hand,  there  is  no  cleavage, 
and  the  fractured  surfaces  of  the  crystals  are  curved 
(or  subconchoidal),  with  a  glassy  or  almost  resinous 
appearance.  This  presence  or  absence  of  cleavage  is 
of  prime  importance,  and  affords  a  very  ready  means 
of  distinguishing  between  crystals  of  calcite  and 
aragonite.  If  we  take  a  thin,  prismatic  or  needle- 
shaped  crystal  (which,  having  been  previously  tested 
with  acid,  we  know  to  be  calcium  carbonate)  and 
snip  off  the  end,  we  can  at  once  say  whether  the  min- 
eral is  calcite  or  aragonite;  if  it  be  calcite,  it  will 
break  along  a  smooth  surface,  set  obliquely  to  the 
length  of  the  prism;  while  if  it  be  aragonite,  there  is 
no  plane  surface  of  fracture. 

Most  frequently,  however,  aragonite  is  met  with 
in  a  very  minutely  crystallized  condition,  forming 
finely  fibrous  compact  masses  of  various  shapes  with 
rounded  outlines.  These  rounded  shapes  may  be 
nodular  or  coralloidal,  and  they  are  often  of  con- 
siderable complexity  and  beauty.  A  coralloidal  (i.e. 
coral-like)  form  is  represented  in  Fig.  i,  this  being 
the  so-called  flos-ferri  (i.e.  flower  of  iron),  which  is 
found  in  the  iron-mines  at  Eisenerz,  in  Styria.  These 
snow-white,  branching  forms  are  often  extremely 
beautiful,  and  they  present  quite  the  appearance 
of  certain  organic  structures.  When  one  of  these 
branches  is  broken  across  it  will  be  seen  to  have  an 
internal  radiated  structure,  due  to  the  very  close 
aggregation  of  numberless  fine,  needle-like  crystals, 
arranged  perpendicular  to  the  axis  of  the  branch. 


ARAGONITE  149 


Another  well-known  form  of  aragonite  is  the  so- 
called  pisolite,  or  pea-stone,  represented  in  Fig.  2, 
which  consists  of  concentric  shells  of  material,  form- 
ing small  balls  about  the  size  of  peas;  numbers  of 
these  balls  are  closely  packed  together,  and  the  spaces 
between  them  filled  with  material  of  the  same  kind, 
to  form  large,  compact  masses.  The  several  layers 
of  the  balls  can  be  readily  peeled  off  like  the  coats  of 
an  onion ;  and  each  layer  when  broken  across  shows 
a  finely  fibrous  structure,  with  the  fibers  arranged 
perpendicular  to  the  surfaces  of  the  shells.  Pisolitic 
aragonite  is  being  deposited  at  the  present  day  by  the 
hot  springs  at  Carlsbad,  in  Bohemia,  the  waters  of 
which  carry  calcium  carbonate  in  solution.  Many 
other  waters,  especially  in  limestone  districts,  contain 
calcium  carbonate  in  solution;  but  at  the  ordinary 
temperature  this  is  deposited  as  calcite  and  not  as 
aragonite.  The  specimen  represented  in  Fig.  2  is 
from  Carlsbad. 

The  finely  fibrous  forms  of  aragonite  just  described 
are  much  more  frequent  than  distinctly  formed  crys- 
tals ;  in  calcite,  on  the  other  hand,  crystals  are  of  more 
common  occurrence.  We  must  not,  however,  assume 
that  all  fibrous  forms  of  calcium  carbonate  are  ara- 
gonite, for  they  are  also  met  with  in  calcite.  Indeed, 
most  stalagmitic  or  onyx  marbles  consist  of  calcite, 
and  not  of  aragonite,  as  often  stated. 

Aragonite  is  both  rather  harder  ( hardness  =  3^- 
4)  and  denser  (sp.  gr.  2.93)  than  calcite;  but  in  finely 
fibrous  and  perhaps  somewhat  cellular  masses  these 
differences  may  not  be  appreciable.  A  doubtful 


150        THE    WORLD'S    MINERALS 

specimen  may,  however,  often  be  determined  by 
floating  a  minute  fragment  in  a  heavy  liquid  together 
with  known  fragments  of  calcite  and  aragonite.  The 
differences  in  the  optical  characters  of  the  two  min- 
erals are  also  of  assistance  for  purposes  of  determi- 
nation. 

Both  calcite  and  aragonite  effervesce  freely  in  cold, 
dilute  hydrochloric  acid;  but  although  these  two 
minerals  are  identical  in  chemical  composition,  it 
does  not  follow  that  they  will  always  show  the  same 
chemical  reactions,  for  they  possess  differences  in 
their  molecular  structure.  For  instance,  if  the  finely 
powdered  material  be  added  to  a  dilute  solution  of 
cobalt  nitrate  and  -this  be  boiled  for  a  few  minutes, 
aragonite  becomes  lilac-red  in  color,  while  calcite 
remains  white. 

CERUSITE 

(Plate  17,  Fig.  4;  Plate  18,  Fig.  4).— Cerusite, 
or  white-lead-ore,  is  carbonate  of  lead  (PbCOs), 
crystallizing  in  the  orthorhombic  system.  The  crys- 
tals are  closely  related  in  form,  angles,  and  twinning 
to  crystals  of  aragonite,  so  that  these  two  minerals, 
together  with  some  others  (namely,  witherite,  or 
barium  carbonate,  and  strontianite,  or  strontium  car- 
bonate), belong  to  the  same  isomorphous  group.  Of 
this  group  of  orthorhombic  carbonates,  aragonite 
may  be  taken  as  the  type,  just  as  calcite  was  taken  as 
the  type  of  the  isomorphous  group  of  rhombohedral 
carbonates  (p.  143).  Now  we  have  already  seen  that 
calcite  and  aragonite  are  dimorphous  (p.  146),  so  we 


CARBONATES. 


Plate  18. 


^/Swr/^roSs 


1 — 3,  Aragonite  (Flos-ferri).    4,  Cenjssit?: 


CERUSITE  151 


have  here  an  excellent  example  of  two  parallel  groups 
of  isomorphous  minerals,  forming  all  together  what 
is  known  as  an  isodimorphous  series. 

Simple  crystals  of  cerusite,  such  as  represented  in 
Plate  17,  Fig.  4,  are  of  comparatively  rare  occur- 
rence. Almost  invariably  the  crystals  are  twinned, 
and  this  twinning  is  indeed  a  very  characteristic  fea- 
ture of  the  mineral.  A  six-rayed  arrangement  of 
platy  crystals  is  very  often  to  be  recognized.  Fibrous 
aggregates  of  needle-like  crystals  (Plate  18,  Fig.  4) 
and  granular  and  compact  forms  are  also  frequent. 

Besides  the  twinning,  cerusite  possesses  several 
other  characteristic  features  which  enable  it  to  be 
readily  identified.  One  of  these  is  the  adamantine 
luster  on  the  bright  crystal  faces;  and  another  is  the 
high  specific  gravity  (6.5) ,  which  is  remarkably  high 
for  a  colorless,  transparent  mineral.  If  we  have  a 
white,  very  heavy,  and  fairly  soft  (H.  =  3/^)  min- 
eral which  effervesces  with  acid,  we  may  be  sure  that 
we  are  dealing  with  cerusite.  The  high  specific 
gravity  is  of  course  due  to  the  presence  of  lead,  and 
beads  of  this  metal  are  readily  obtained  by  heating 
the  mineral  on  charcoal  before  the  blowpipe. 

Cerusite  occurs  in  the  upper  oxidized  zones  of 
metalliferous  veins,  having  resulted  by  the  action  of 
carbonated  waters  on  galena.  In  Plate  18,  Fig.  4, 
the  white  fibrous  cerusite  is  associated  with  brown 
limonite,  the  latter  being  a  product  of  alteration  of 
iron-pyrites;  we  have  here  two  secondary  minerals 
produced  by  the  alteration  of  sulphide  minerals  in  a 
metalliferous  vein. 


152        THE    WORLD'S    MINERALS 

When  met  with  in  large  quantities  cerusite  is  an 
important  ore  of  lead,  containing,  when  pure,  83.5 
per  cent,  of  the  metal.  It  is  found  in  upper  levels  of 
most  lead-mines,  and  is  thus  a  fairly  common  min- 
eral. Very  fine  specimens  have  been  found  at  Broken 
Hill  in  New  South  Wales  and  at  Broken  Hill  in 
Northwestern  Rhodesia.  The  unusually  fine  crystal- 
lized specimen  represented  in  Plate  17,  Fig.  4,  is 
from  Badenweiler,  in  Baden;  and  that  in  Plate  18, 
Fig.  4,  is  from  Monte  Vecchio,  in  Sardinia. 

CHESSYLITE 

(Plate  19,  Figs,  i  and  2). — In  Plate  19  are  rep- 
resented two  minerals,  each  consisting  of  copper  car- 
bonate combined  with  an  excess  of  copper  oxide  and 
some  water;  that  is,  they  are  hydrated  basic  copper 
carbonates.  They  thus  contain  the  same  chemical 
elements;  but  these  are  combined  together  in  dif- 
ferent proportions,  and  we  have,  in  fact,  two  quite 
distinct  chemical  compounds.  The  formula  of  ches- 
sylite  is  2CuCO3.Cu(OH)2,  or  3CuO.2CO2.H2O; 
while  that  of  malachite  is  CuCO3.Cu(OH)2,  or 
2CuO.CO2.Hi5O,  there  being  rather  more  copper  ox- 
ide and  water  in  the  latter.  When  pure,  the  two 
minerals  contain  respectively  55.2  and  57.4  per  cent, 
of  metallic  copper. 

Although  there  is  only  a  slight  difference  in  the 
chemical  composition  of  these  two  minerals,  yet  they 
present  a  striking  difference  in  their  external  appear- 
ance. Chessylite  is  of  a  bright  blue  color,  while 


CHESSYLITE  153 

malachite  is  emerald-green.  They  are  both  soft  min- 
erals (H.  =  3/^-4) ,  and  can  be  readily  scratched  with 
a  knife;  and  being  carbonates  they  both  effervesce 
with  acid,  giving  a  green  copper  solution.  Further, 
they  frequently  occur  together,  having  been  produced 
by  the  alteration,  due  to  the  action  of  carbonated 
waters,  of  other  copper-bearing  minerals,  particu- 
larly copper-pyrites. 

Chessylite  receives  its  name  from  the  locality — 
Chessy,  near  Lyons,  in  France — whence  the  best 
crystallized  specimens  are  obtained.  Another  name 
also  in  common  use  for  this  species  is  azurite,  in 
allusion  to  the  characteristic  azure-blue  color  of  the 
mineral.  It  crystallizes  in  the  monoclinic  system, 
but  although  it  nearly  always  occurs  as  crystals,  these 
do  not,  as  a  rule,  present  any  very  characteristic 
form.  In  Fig.  i  are  shown  divergent  groups  of  small 
prismatic  crystals,  and  in  Fig.  2  small,  bright  crystals 
are  thickly  clustered  on  the  surface  of  the  matrix; 
both  these  specimens  are  from  Chessy.  Good  crystal- 
lized specimens  of  chessylite  are  also  found  in  the 
Copper  Queen  mine  and  in  other  copper-mines  in 
Arizona,  and  at  Broken  Hill  in  New  South  Wales. 

Although  crystals  are  the  most  frequent,  this  min- 
eral is  also  found  as  powdery  or  earthy  masses  of  a 
bright  sky-blue  color,  the  color  here  being  less  dark 
than  in  the  crystals.  The  streak,  or  powder,  of  the 
crystals  is  also  sky-blue. 

As  already  mentioned,  chessylite  frequently  occurs 
in  association  with  malachite,  and  in  Figs,  i  and  2  are 
to  be  seen  patches  of  green  malachite  with  the  blue 


154        THE   WORLD'S    MINERALS 

chessylite.  In  Arizona  the  two  minerals  are  some- 
times banded  together  in  compact  masses,  and  speci- 
mens of  this  kind,  when  cut  and  polished,  find  a 
limited  application  in  cheap  jewelry. 

MALACHITE 

(Plate  19,  Figs.  3  and  4). — While  chessylite  is 
usually  found  as  crystals,  on  the  other  hand  crystals 
of  malachite  are  of  rare  occurrence.  These  are  never 
very  distinctly  developed,  usually  having  the  form  of 
fine  needles  grouped  together  in  silky  tufts.  Ordi- 
narily the  mineral  occurs  as  compact  masses  (Fig.  3) , 
which  may  sometimes  present  rounded  nodular 
(mamillary)  surfaces  (Fig.  4).  These  nodular  masses 
when  broken  open  or  when  cut  and  polished  (as  in 
Fig.  4)  exhibit  a  concentric  arrangement  of  lighter 
and  darker  green  bands,  together  with  a  more  or  less 
distinct  radially  fibrous  structure. 

Malachite  is  a  much  more  common  mineral  than 
chessylite,  and  it  is  the  more  stable  of  the  two.  This 
is  proved  by  the  fact  that  we  often  find  crystals  of 
chessylite  altered  to  malachite — that  is,  pseudo- 
morphs  of  malachite  after  chessylite.  The  green 
stains  to  be  observed  on  the  stones  and  ore  from 
all  copper-mines  consist  of  malachite,  this  having 
been  formed  by  the  action  of  weathering  agents  en 
the  other  copper-bearing  minerals  of  the  ore.  The 
green  patina  on  bronze  is  also  due  to  the  surface  al- 
teration to  malachite  of  the  copper  contained  in  the 
bronze;  and  on  ancient  bronze  implements  which 


CARBONATES. 


Plate  19. 


V 


1,  2,  Azurite.    3,  4,  Malachite; 


MALACHITE  155 


have  long  been  buried  in  the  soil,  nodular  concretions 
of  malachite  are  sometimes  to  be  seen. 

Large  masses  of  compact  malachite,  sometimes 
weighing  several  tons,  are  found  in  the  copper-mines 
of  the  Ural  Mountains  (Fig.  4),  at  Burra-Burra  in 
South  Australia,  and  in  Arizona.  Such  material, 
especially  that  from  Russia,  is  cut  and  polished  for 
ornamental  purposes,  being  made  into  vases  and  other 
small  articles,  or  used  for  inlaying  in  table-tops  and 
columns.  The  material  is  easily  worked  and  may 
be  turned  in  the  lathe,  and  it  takes  a  very  good  polish. 
When  found  in  sufficiently  large  quantities,  mala- 
chite is  also  of  importance  as  an  ore  of  copper. 


CHAPTER  X 

THE  SULPHATES,  CHROMATES,  MOLYB- 
DATES,  AND  TUNGSTATES 

IN  this  group  of  oxygen-salts  are  brought  together 
those  minerals  which  contain  in  their  acid  portion 
the  chemical  elements  sulphur,  chromium,  molyb- 
denum, or  tungsten,  all  of  which,  with  the  exception 
of  sulphur,  are  themselves  metals.  These  are  closely 
related  elements,  falling  in  the  sixth  group  of  the 
chemists'  periodic  classification.  Their  salts  can  all 
be  expressed  by  a  similar  chemical  formula,  which 
may  be  written  generally  as  R"MO4  when  R"  is  the 
metal  and  MO4  the  acid  portion,  M  standing  for  one 
or  other  of  the  four  elements  named  above.  Some 
of  the  salts  may  contain  in  addition  water  of  crystal- 
lization. 

The  sulphates  are  the  most  important  and  numer- 
ous members  of  this  group.  These  are  combinations 
of  a  metal  with  the  well-known  sulphuric  acid,  or 
oil  of  vitriol.  Thus,  the  mineral  anglesite  is  lead 
sulphate,  and  the  general  formula  given  above  here 
becomes  PbSO4.  This  formula  is  similar  to  that  of 
galena  (PbS)  with  the  addition  of  four  atoms  of 
oxygen,  and  it  is  interesting  to  note  that  anglesite  is 
formed  in  nature  by  the  weathering  and  oxidation  of 
galena. 

156 


BARYTES  157 


All  the  minerals  here  considered,  with  the  excep- 
tion of  gypsum  (which  contains  water  of  crystal- 
lization), are  very  heavy.  They  are  often  trans- 
parent and  well  crystallized,  and  are  sometimes 
brightly  colored.  Most  of  them  are  of  economic 
importance. 

BARYTES 

(Plate  20,  Figs,  i  and  2). — This  is  the  sulphate  of 
barium  (BaSCh).  It  is  a  common  mineral,  and  one 
which  is  very  often  found  as  large,  well-developed 
crystals.  The  crystals  belong  to  the  orthorhombic 
system,  and  they  vary  considerably  in  their  habit, 
being  tabular  (Fig.  i)  or  prismatic  (Fig.  2).  The 
rhomb-shaped  tabular  crystals  shown  in  Fig.  i  are 
bounded  by  the  large  basal  plane,  perpendicular  to 
which  are  narrow  faces  of  a  rhombic  prism  forming 
the  edges  of  the  plates.  The  angles  between  the 
prism  faces  are  78^°  and  ioij^°,  these  being  also 
the  plane  angles  of  the  rhomb  forming  the  basal 
plane.  The  crystal  shown  in  Fig.  2  is  bounded  by 
the  same  faces,  with  the  addition  of  a  macrodome. 
Here  the  crystal  is  elongated  in  the  direction  of  one 
of  the  horizontal  crystallographic  axes — the  macro- 
axis,  which  in  the  picture  is  set  up  vertically.  The 
two  faces  at  the  top  of  the  crystal  (one  of  them  at  the 
back,  and  therefore  not  visible  in  the  picture),  which 
slope  away  on  either  side  from  the  upper  line  or 
ridge,  are  the  faces  of  the  rhombic  prism,  with  an 
angle  of  78^°.  The  long,  narrow  face  in  the  front, 
and  extending  to  the  summit  of  the  crystal,  is  the 


158        THE    WORLD'S    MINERALS 

basal  plane;  and  the  two  long  faces  on  either  side  of 
this  belong  to  a  macrodome.  At  the  lower  end  of 
this  crystal  are  to  be  seen  a  number  of  smaller  crys- 
tals grown  in  parallel  position  with  the  main  crystal. 

A  very  important  character  of  barytes  is  its  cleav- 
age, there  being  three  directions  in  which  the  crystals 
may  be  readily  split.  The  best  cleavage  is  parallel 
to  the  basal  plane,  and  on  this  surface  the  crystals 
frequently  exhibit  a  pearly  appearance,  owing  to  the 
presence  of  cleavage  cracks  within  the  crystal.  The 
two  other  rather  less  perfect  cleavages  are  parallel 
to  the  two  pairs  of  parallel  prism  faces;  in  the  pic- 
tures, especially  in  Fig.  2,  these  cleavages  are  shown 
as  cracks,  running  across  the  crystal  parallel  to  the 
prism  faces.  Any  crystal  of  barytes  can  be  easily 
broken  along  these  three  cleavages  into  rhomb-shaped 
plates,  similar  to  the  crystals  in  Fig.  i.  We  have 
already  seen  that  calcite  also  possesses  three  direc- 
tions of  perfect  cleavage;  but  there  is  an  important 
difference  between  the  cleavage  of  calcite  and  barytes. 
In  calcite  the  angle  between  any  two  of  the  cleav- 
ages is  always  the  same;  while  in  barytes  the  prism 
cleavages  enclose  an  angle  of  78^°,  and  these  are 
each  at  90°  (i.e.  at  right  angles)  to  the  basal  cleavage. 

Another  character  of  importance  is  the  high  spe- 
cific gravity  (4.5),  and  on  this  account  the  mineral 
is  commonly  known  as  heavy-spar.  The  hardness  is 
about  the  same  as  that  of  calcite;  so  that  the  mineral 
can  be  readily  scratched  with  a  knife. 

Barytes  is  often  white  or  colorless,  or  various 
shades  of  yellow  or  brown;  and  the  crystals  may  be 


BARYTES  159 

quite  transparent,  or  only  translucent  to  opaque.  In 
addition  to  crystals,  platy  or  granular  masses,  show- 
ing larger  or  smaller  cleavage  surfaces  when  broken 
across,  are  of  frequent  occurrence. 

In  their  general  appearance,  cleavage  masses  of 
white  barytes  are  not  unlike  calcite;  but  the  two 
minerals  may  be  readily  distinguished  by  their 
weight,  by  the  angles  between  their  cleavages,  and 
by  the  fact  that  barytes  does  not  effervesce  with  acids. 

Barytes  is  usually  found  in  veins,  either  alone  or 
more  often  in  association  with  metalliferous  ores,  es- 
pecially ores  of  lead,  and  it  is  thus  a  common  mineral 
in  many  mining  districts.  The  specimen  represented 
in  Plate  20,  Fig.  i,  showing  crystals  of  barytes  resting 
on  and  partly  penetrated  by  needles  of  stibnite  (anti- 
mony-ore), is  from  the  mines  at  Felsobanya,  in 
Hungary.  The  one  shown  in  Fig.  2  is  from  the 
iron  (red  haematite)  mines  at  Frizington,  near 
Whitehaven,  in  Cumberland,  where  large  and 
beautifully  crystallized  specimens  are  abundant. 

Barytes  is  used  commercially  for  the  manufac- 
ture of  white  paint,  for  which  purpose  it  is  often 
mixed  with  white  lead.  It  is  also  used  for  giving 
weight  and  finish  to  certain  kinds  of  paper.  Much 
of  the  material  mined  under  the  name  of  barytes  is 
really  the  more  valuable  mineral  ivitherite,  or  car- 
bonate of  barium,  and  not  the  sulphate.  This,  being 
soluble  in  acids,  is  more  readily  converted  into  vari- 
ous barium  compounds,  which,  amongst  other  uses, 
are  employed  in  sugar-refining  and  for  making  rat 
poison. 


160         THE    WORLD'S    MINERALS 


CELESTITE 

(Plate  20,  Fig.  4). — This  is  strontium  sulphate 
(SrSCX).  In  the  form,  angles,  and  cleavages  of  its 
orthorhombic  crystals  it  presents  the  greatest  similar- 
ity to  barytes.  These  two  minerals  are,  in  fact,  iso- 
morphous ;  and  belonging  to  the  same  group  we  also 
have  the  next  mineral  to  be  considered — namely, 
anglesite,  or  lead  sulphate. 

In  Fig.  4  we  have  a  parallel  grouping  of  crystals 
of  prismatic  habit.  These  crystals  are  terminated  at 
their  upper  ends  by  the  prism  faces,  parallel  to  which 
are  perfect  cleavages,  enclosing  an  angle  of  76° 
(corresponding  to  the  angle  of  78^°  in  barytes). 
The  vertical  faces  in  the  picture  belong  to  a  brachy- 
dome,  and  the  small  triangular  faces  on  the  corners 
belong  to  a  macrodome. 

A  characteristic,  though  not  constant,  feature  of 
celestite  is  a  faint  bluish  shade  of  color  (not  greenish 
as  in  picture).  It  is  on  this  account  that  the  mineral 
receives  its  name,  although  the  color  is  never  a  deep 
sky-blue.  White  and  yellowish  crystals  are  also 
common. 

Celestite  is  rather  lighter  (sp.  gr.  4.0)  than  barytes, 
but  its  hardness  is  about  the  same.  The  best  methods 
of  distinguishing  between  these  two  minerals  is  af- 
forded by  the  colors  they  impart  to  the  non-luminous 
flame  of  a  Bunsen-burner.  A  fragment  of  the  min- 
eral, moistened  with  hydrochloric  acid,  is  supported 
on  platinum  wire  in  the  flame,  when  celestite  gives 


SULPHATES. 


Plate  20. 


1,  2,  Barytes.    3,  Anglesite.    4,  Celestite. 


CELESTITE— ANGLESITE  161 

an  intense  crimson  color  and  barytes  a  pale  yellowish- 
green,  these  being  the  characteristic  flame  colorations 
of  all  strontium  and  barium  compounds  respectively. 
On  this  property  depends  the  use  of  celestite  for 
producing  the  red  fire  of  pyrotechnic  displays.  An- 
other use  for  the  strontium  compounds  obtained  from 
celestite  is  in  sugar-refining. 

Fine  crystallized  specimens  of  celestite  are  abun- 
dant in  the  red  marls  of  Triassic  age  in  the  neighbor- 
hood of  Bristol.  Good  crystals  are  also  common  in 
the  sulphur-mines  near  Girgenti,  in  Sicily  (Fig.  4)  ; 
and  very  large  bluish  crystals  are  found  in  a 
limestone  cave  on  Strontian  Island  in  Lake  Erie. 

ANGLESITE 

(Plate  20,  Fig.  3).— Sulphate  of  lead  (PbSCX)  is 
another  orthorhombic  mineral,  belonging  to  the  same 
isomorphous  group  with  barytes  and  celestite.  Its 
crystals  are  usually  small  and  complex  in  form,  and 
they  are  not  always  readily  determined  by  mere  in- 
spection. The  three  crystals  on  the  matrix  in  Fig.  3 
are  unusually  perfect.  Often  the  crystals  are  color- 
less and  transparent,  with  a  very  brilliant  and 
adamantine  luster.  Being  a  lead  mineral,  it  is  very 
heavy  (sp.  gr.  6.3). 

Anglesite  is  found  in  the  upper  weather  portions 
of  lead-bearing  veins,  where  it  has  been  formed  by 
the  alteration  of  galena.  In  the  old  mine  on  Parys 
Mountain,  in  the  island  of  Anglesey,  minute  crystals 
were  found  in  abundance,  encrusting  the  surface  of 


162         THE    WORLD'S    MINERALS 

cellular  limonite,  the  latter  being  also  a  mineral  of 
secondary  origin.  It  is  from  this  locality  that  the 
mineral  receives  its  name.  Good  crystals  have  also 
been  found  in  the  lead-mines  of  Derbyshire,  at 
Monteponi  in  Sardinia  ( Fig.  3 ) ,  Broken  Hill  in  New 
South  Wales,  and  in  Tasmania. 

GYPSUM 

(Plate  21,  Figs.  1-3). — This  is  a  common  mineral 
of  considerable  practical  importance.  It  is  composed 
of  calcium  sulphate,  with  two  molecules  of  water  of 
crystallization,  the  formula  being  CaSO4.2H2O. 

Bearing  this  composition  in  mind,  it  is  of  interest 
to  trace  the  chemical  processes  by  which  gypsum  is 
formed  in  nature.  By  the  weathering  of  iron-pyrites, 
a  mineral  which  as  scattered  grains  and  crystals  oc- 
curs in  rocks  of  almost  every  kind,  sulphuric  acid  or 
iron  sulphate  is  produced,  and  these  compounds,  be- 
ing soluble  in  water,  may  be  transported  from  their 
place  of  origin.  Now  if  such  solutions  come  into 
contact  with  limestone  rocks  or  marls  containing 
some  calcium  carbonate,  a  reaction  takes  place,  the 
calcium  of  the  rock  combining  with  the  sulphuric 
acid  to  form  calcium  sulphate.  This  new  compound, 
being  only  slightly  soluble  in  water,  may  be  deposited 
as  crystals  of  gypsum;  but  if  a  large  excess  of  water 
is  present,  the  whole  of  the  calcium  sulphate  may  be 
carried  away  in  solution.  In  the  latter  case  a  hard 
water  results,  and  one  of  which  the  hardness  is  per- 
manent, since  it  cannot  be  removed  by  boiling  or  by 


GYPSUM  163 


the  addition  of  lime,  as  may  be  done  with  a  water 
rendered  hard  by  the  presence  of  calcium  carbonate. 
Water  containing  much  calcium  sulphate  in  solution 
may  under  certain  conditions  collect  in  lakes,  and 
with  the  gradual  evaporation  of  the  water  gypsum 
may  be  deposited  on  the  bed  of  the  lake.  By  such  a 
process  vast  beds  of  gypsum  have  been  deposited  in 
inland  seas  in  past  geological  epochs,  and  it  is  such 
beds  of  gypsum  that  are  quarried  at  the  present  day. 

The  formation  of  gypsum  can  readily  be  demon- 
strated experimentally.  A  fragment  of  calcite  is  dis- 
solved in  a  drop  of  hydrochloric  acid  placed  on  a 
microscope  slide,  and  to  the  solution  of  calcium 
chloride  so  obtained  a  drop  of  dilute  sulphuric  acid, 
or  a  fragment  of  any  soluble  sulphate,  is  added.  The 
drop  is  then  allowed  to  evaporate,  when,  under  the 
microscope,  crystals  of  gypsum  may  be  observed  in 
the  process  of  growth.  These  crystals  have  the  form 
of  slender  needles,  with  oblique  terminations  (as  in 
Plate  21,  Fig.  i),  and  they  arrange  themselves  in 
pretty  star-like  groups. 

Well-shaped  crystals  of  gypsum  are  of  common  oc- 
currence in  nature,  and  when  found  embedded  in 
clay;  as  is  very  frequently  the  case,  they  are  bounded 
on  all  sides  by  crystal-faces.  The  crystals  represented 
in  Plate  21,  Fig.  i,  being  attached  to  the  matrix,  show 
faces  at  one  end  only.  Gypsum  crystallizes  in  the 
monoclinic  system,  and  the  crystals  possess  only  one 
plane  of  symmetry.  The  most  usual  form  is  as  shown 
in  Fig.  i ;  here  the  two  larger  crystals  are  tabular  in 
habit  parallel  to  the  plane  of  symmetry,  while  the 


164        THE    WORLD'S    MINERALS 

smaller  crystals  of  the  group  are  prismatic  in  habit. 
The  faces  present  belong  to  clino-pinacoid  (a  pair 
of  faces  both  parallel  to  the  plane  of  symmetry),  a 
rhombic  prism  (the  two  narrow  faces  placed  ver- 
tically in  the  crystal  forming  the  center  of  the  group) , 
and  a  pyramid  or  dome  (the  two  narrow  faces  placed 
obliquely  at  the  top  of  the  crystal). 

Crystals  of 'gypsum  are  often  twinned,  two  crystals 
being  grown  together  in  such  a  position  that  they  are 
symmetrical  about  a  plane  which  truncates  the  front 
edge  of  the  prism  in  Fig.  i.  Such  a  twinned  crystal 
is  shown  in  Fig.  2. 

Cleavage  is  a  very  important  character  of  gypsum; 
the  crystals  can  be  split  with  great  ease  into  thin 
leaves  parallel  to  the  plane  of  symmetry  or  clino- 
pinacoid.  On  this  surface  the  luster  is  often  pearly 
in  character,  and  there  are  frequently  to  be  seen 
brightly  colored  bands  of  the  same  nature  as  New- 
ton's rings.  The  crystals  also  possess  a  characteristic 
fibrous  cleavage  parallel  to  the  pair  of  dome  faces, 
this  cleavage  being  often  shown  by  silky  bands  run- 
ning obliquely  across  the  surface  of  the  perfect  cleav- 
age first  mentioned. 

Crystals  of  gypsum  are7  usually  colorless  and 
transparent,  and  only  rarely  do  they  show  the  yellow- 
ish colors  often  seen  in  massive  gypsum.  The  name 
selenite,  sometimes  used  for  this  species,  is  more 
correctly  applied  to  the  transparent  crystals;  this 
name  refers  to  the  somewhat  fanciful  resemblance 
of  the  luster  of  the  crystals  to  moonlight. 

Another  character  of  importance  is  the  low  degree 


Plate  21. 


1—  3,  Gypsum.    4,  Linarite. 


GYPSUM  165 


of  hardness  (H.  —  2).  Gypsum  is  one  of  the  few 
common,  crystallized  minerals  that  can  be  scratched 
with  the  finger-nail.  The  specific  gravity  (2.3)  is 
also  low.  Other  light  and  very  soft  minerals  with  a 
perfect  cleavage,  which  may  perhaps  be  mistaken 
for  gypsum,  are  mica  and  talc.  Cleavage  flakes  of 
gypsum,  though  flexible,  are  not  elastic  like  mica,  nor 
greasy  to  the  touch  like  talc ;  and,  further,  they  always 
show,  when  bent,  the  secondary  fibrous  cleavage. 

Besides  occurring  as  crystals,  gypsum  is  frequently 
found  as  granular  masses,  forming  enormous  beds. 
Such  material  when  fine-grained  and  compact  is  the 
well-known  alabaster,  which,  owing  to  the  softness 
of  the  mineral,  particularly  lends  itself  to  carving. 
Fibrous  masses  are  also  common  as  a  filling  of  veins, 
the  fine  fibers  being  arranged  perpendicularly  to  the 
walls  of  the  vein  (Fig.  3).  Such  material  (satin- 
spar),  when  cut  and  polished,  with  a  rounded  sur- 
face, displays  a  satiny  luster  like  catVeye,  and  it  is 
used  for  making  beads  and  other  small  ornaments. 

Good  crystallized  specimens  of  gypsum  are  found 
at  very  many  places,  and  detached  crystals  are  to  be 
found  embedded  in  most  clays.  The  finest  crystals 
are  those  from  the  salt-mines  at  Bex,  in  Switzer- 
landr  and  the  sulphur-mines  near  Girgenti,  in  Sicily. 
Large  numbers  of  enormous  crystals,  a  yard  in  length, 
have  been  found  lining  a  cave  in  Utah.  Extensive 
beds  of  massive  gypsum  are  quarried  in  the  neigh- 
borhood of  Paris,  in  Nottinghamshire  and  Stafford- 
shire, and  many  other  places.  The  material  is  heated 
in  kilns  to  drive  off  a  portion  of  the  water  of 


166         THE    WORLD'S    MINERALS 

crystallization,  and  thereby  converted  into  the  well- 
known  plaster  of  Paris. 

LINARITE 

(Plate  21,  Fig.  4). — Linarite  is  a  very  attractive 
though  quite  rare  mineral,  and  one  which  is  found 
at  only  a  few  localities.  It  is  a  basic  sulphate  of  lead 
and  copper  of  complex  composition,  the  brilliant 
sky-blue  color  being  due  to  the  presence  of  the  latter 
element.  The  crystals  are  small,  and  have  a  bright 
luster  as  well  as  a  bright  color;  in  general  appear- 
ance they  are  not  unlike  crystals  of  chessylite.  The 
specimen  represented  in  Fig.  4  is  from  the  old  lead 
and  copper  mines  of  Roughten  Gill,  near  Keswick, 
in  Cumberland.  Specimens  have  also  been  found 
at  Leadhills,  in  Scotland,  but,  curiously,  none  at 
Linares,  in  Spain,  from  which  place  the  mineral 
receives  its  name. 

CROCOITE 

(Plate  22,  Fig.  i).— Chromate  of  lead  (PbCrO*) 
occurs  in  nature  as  beautiful  crystals  with  a  bright 
hyacinth-red  color  and  brilliant  luster,  which  in  gen- 
eral appearance  are  not  unlike  crystals  of  the  artificial 
salt  potassium  bichromate.  Unfortunately  the  crys- 
tals lose  their  luster  and  become  dull  on  exposure 
to  light,  and  in  collections  it  is  therefore  necessary 
to  keep  them  under  cover.  The  crystals  belong  to 
the  monoclinic  system,  and  are  usually  prismatic  in 
habit.  In  the  picture  (Fig.  i)  are  shown  several 


CROCOITE— WOLFRAMITE          167 

prismatic  crystals  irregularly  grouped  on  the  surface 
of  a  piece  of  yellowish  quartz.  This  specimen  is 
from  Beresov,  in  the  Ural  Mountains,  where  the  min- 
eral is  found,  together  with  galena,  in  veins  of 
gold-bearing  quartz.  Magnificent  groups  of  pris- 
matic crystals  several  inches  in  length  have  recently 
been  found  in  a  lead-mine  near  Dundas,  in  Tasmania. 
Crocoite  is  a  mineral  of  secondary  origin,  having 
been  formed  by  the  alteration  of  galena. 

WOLFRAMITE 

(Plate  22,  Fig.  2). — This  mineral  is  a  tungstate 
of  iron  and  manganese,  the  two  isomorphous  mole- 
cules FeWCX  and  MnWO*  being  mixed  together  in 
variable  proportions,  so  that  the  formula  may  be 
written  as  (Fe,Mn)WO4.  Here  W  is  the  chemical 
symbol  of  the  metal  tungsten,  or  wolfram.  The 
mineral  is  very  heavy  (sp.  gr.  7.1  to  7.5,  varying  with 
the  proportion  of  iron  and  manganese),  and  opaque, 
with  a  dark  brownish-black  or  pitch-black  color  and 
a  sub-metallic  luster. 

Crystals  of  wolframite  are  rarely  met  with,  while 
fine  isolated  crystals,  such  as  that  shown  in  Fig.  2, 
are  quite  exceptional.  They  are  monoclinic,  and 
possess  a  perfect  cleavage  in  one  direction  parallel 
to  the  plane  of  symmetry.  This  perfect  cleavage  is 
always  to  be  seen  in  the  more  commonly  occurring 
granular  or  columnar  masses,  such  as  are  often  found 
embedded  in  quartz. 

Wolframite  often  occurs  in  association  with  tin- 


168        THE    WORLD'S    MINERALS 

stone  (cassiterite) — for  instance,  in  Cornwall,  and  at 
Zinnwald  in  Bohemia,  the  crystal  in  Fig.  2  being 
from  the  latter  locality.  Being  both  very  heavy 
minerals,  their  separation  is  rather  a  troublesome 
operation  for  the  tin-miner. 

Sodium  tungstate,  prepared  by  fusing  wolframite 
with  soda,  is  employed  in  dyeing  and  for  rendering 
fabrics  non-inflammable.  Metallic  tungsten  is  used 
for  the  manufacture  of  the  very  hard  and  tough  tung- 
sten-steel ;  and  quite  recently,  in  the  form  of  fine  wire, 
it  has  been  used  for  the  filaments  of  the  so-called 
"osram"  electric  lamps. 

WULFENITE 

(Plate  22,  Fig.  3). — This  name  is  not  to  be  con- 
fused with  the  name  of  the  mineral  last  described. 
It  was  given  in  honor  of  the  eighteenth-century  Aus- 
trian mineralogist  Wulfen;  while  wolfram  is  an  old 
German  mining  term  meaning  "wolf  froth."  Wul- 
fenite  is  a  molybdate  of  lead  (PbMoCX),  crystal- 
lizing in  the  tetragonal  system.  The  crystals  almost 
always  have  the  form  of  square  plates  with  bevelled 
edges,  and  sometimes  the  corners  also  are  replaced  by 
narrow  pyramidal  faces  (Fig.  3).  The  crystals  are 
often  of  a  yellowish  or  greyish  color,  but  sometimes 
they  are  bright  orange-yellow  or  orange-red,  with  a 
brilliant  luster.  The  mineral  is  one  of  secondary 
origin,  having  been  formed  by  the  alteration  of  ga- 
lena in  veins  of  lead-ore.  The  earliest  known  local- 
ity, and  one  that  has  produced  a  large  number  of 


CHROMATES,  TUNGSTATES,  &c. 


Plate  22. 


1,  Crocoite.    2,  Wolframite.    3,  Wulfenite.    4,  Scheelite. 


WULFENITE— SCHEELITE          169 

good  specimens,  is  Bleiberg  (meaning,  in  German, 
"lead  mountain"),  in  Carinthia;  while  very  fine 
groups  of  crystals  of  richer  colors  have  been  found 
in  Arizona  (Fig.  3)  and  Utah. 

SCHEELITE 

(Plate  22,  Fig.  4). — It  was  in  this  mineral  that 
the  Swedish  chemist  Scheele  discovered,  in  the  year 
1781,  the  element  tungsten.  The  mineral  itself  had 
previously  been  known  as  tungsten,  which  means,  in 
Swedish,  "heavy  stone,"  and  it  was  afterwards  re- 
named scheelite  in  honor  of  Scheele.  Chemically,  it 
is  a  tungstate  of  calcium  (CaWO4),  and  it  is  iso- 
morphous  with  the  molybdate  of  lead,  wulfenite. 
Its  crystals  are  tetragonal;  but  instead  of  being  tabu- 
lar in  habit  like  crystals  of  wulfenite,  they  are 
pyramidal,  having  the  form  of  square  pyramids.  The 
crystals  are  white,  greyish,  or  yellowish  in  color,  and 
sometimes  almost  transparent.  To  all  appearance 
they  might  be  expected  to  be  quite  light,  but  when 
a  crystal  of  scheelite  is  handled  it  will  be  found  to  be 
surprisingly  heavy  (sp.  gr.  6.0). 

Scheelite  often  occurs  in  association  with  wolfram- 
ite and  tin-stone,  and  it  is  also  met  with  in  granitic 
veins,  sometimes  together  with  gold.  Good  crystals 
have  been  found  at  Tavistock,  in  Devonshire,  and  at 
Carrock  Fell,  in  Cumberland.  The  picture  (Fig.  4; 
from  the  tin-mines  at  Zinnwald,  Bohemia)  shows  a 
number  of  small  crystals  of  the  characteristic  form 
and  color  scattered  over  the  surface  of  a  matrix  of 
quartz. 


CHAPTER  XI 

THE  PHOSPHATES,  ARSENATES,  AND 
VANADATES 

THE  phosphates,  which  are  by  far  the  most  impor- 
tant members  of  this  group,  are  salts  of  phosphoric 
acid,  the  chemical  formula  of  which  is  HsPO*.  The 
non-metallic  and  highly  inflammable  element  phos- 
phorus is  here  combined  with  oxygen  and  hydrogen, 
and  when  this  hydrogen  is  replaced  by  a  metal  we 
have  a  salt  called  a  phosphate. 

Phosphates  are  of  great  importance  in  the  economy 
of  nature,  for  they  enter  into  the  composition  of 
plants  and  animals,  and  bones  are  composed  largely 
of  calcium  phosphate  together  with  calcium  car- 
bonate. These  phosphates  are  all  indirectly  of  min- 
eral origin,  having  been  extracted  from  the  soil  by 
growing  vegetation.  On  this  account  the  mineral 
apatite,  to  be  presently  described,  is  largely  used  in 
the  manufacture  of  artificial  manures,  or  fertilizers. 
Another  phosphate — namely,  turquoise — is  a  valu- 
able precious  stone;  while  still  another  (pyromor- 
phite)  is  an  ore  of  lead.  We  thus  see  that  the  mineral 
phosphates  are  of  considerable  importance.  Many 
of  the  numerous  species,  of  which  no  mention  need 
be  made  in  this  book,  are  of  quite  rare  occurrence, 
and  of  interest  only  to  the  mineralogist  and  crystal- 

170 


APATITE  171 

lographer.  In  addition  to  phosphate,  several  of  the 
minerals  of  this  group  contain  water  of  crystalliza- 
tion, or  some  other  constituent. 

Closely  related  to  the  phosphates,  and  strictly 
isomorphous  with  them,  we  have  the  arsenates,  anti- 
monates,  and  vanadates.  In  these  salts  the  place  of 
phosphorus  is  taken  by  the  chemically  related  ele- 
ments arsenic,  antimony,  or  vanadium,  which,  though 
metallic  elements,  here  play  the  part  of  acids.  The 
chemical  formula  of  all  the  salts  is  of  the  same  type; 
for  instance,  arsenic  acid  is  H3AsO4,  and  vanadic 
acid  is  H3VO4. 

APATITE 

(Plate  23,  Fig.  i). — The  name  apatite  means,  in 
Greek,  "to  deceive,"  because  this  mineral  is  often 
mistaken  for  other  species;  indeed,  some  eminent 
mineralogists  have  described  as  new  minerals  ma- 
terial which  afterwards  proved  to  be  merely  apatite. 
The  name  also  suggests  a  certain  class  of  humor;  the 
announcement  a  few  years  ago  of  an  American 
dealer  ran:  "Minerals  for  presents:  send  your  friend 
an  'apatite'  for  his  Christmas  dinner." 

In  its  chemical  composition  apatite  is  some- 
what complex;  it  is  essentially  a  phosphate  of 
calcium,  but  in  combination  with  this  we  have  a 
small  proportion  of  calcium  fluoride,  or  calcium 
chloride,  and  the  formula  becomes  3Ca3(PO4)2.CaF2, 
or  (CaF)Ca4(PO4)3.  This  is  fluor-apatite,  and  the 
formula  of  the  corresponding  compound  chlor-apa- 
tite  is  3Ca3(PO4)2.CaCl2.  We  thus  have  two  va- 


172        THE    WORLD'S    MINERALS 

rieties  of  apatite,  but  these  can  only  be  distinguished 
by  chemical  tests. 

Apatite  is  often  found  beautifully  crystallized. 
The  crystals  have  the  form  of  a  hexagonal  prism, 
terminated  at  each  end  by  six-sided  basal  planes 
placed  at  right  angles  to  the  six  faces  of  the  prism. 
The  prism  may  be  either  short  (Fig.  i )  or  long  (as  in 
Fig.  3,  of  pyromorphite),  and  the  habit  of  crystals, 
therefore,  either  tabular  or  prismatic.  Occasionally 
the  edges  between  the  prism  and  the  base  are  re- 
placed by  narrow  faces  of  hexagonal  pyramids,  and 
the  corners  by  other  small  facets.  The  faces  on  the 
corners  are  arranged  in  a  peculiar  manner,  which  is 
very  characteristic  of  crystals  of  apatite;  they  are 
present  on  one  side,  not  both  sides,  of  each  edge  of 
the  prism,  and  the  crystals  are  therefore  said  to  be 
hemihedral. 

Crystals  of  apatite  may  be  colorless  and  transparent 
or  white  and  opaque;  often  they  are  of  a  greenish 
or  brownish  shade  of  color,  or  sometimes  they  are 
sky-blue  or  violet  (Fig.  i).  The  crystals  possess  no 
cleavage,  and  their  fracture  is  sub-conchoidal.  In 
addition  to. crystals,  compact  and  earthy  masses  are 
of  abundant  occurrence.  This  form  of  the  mineral 
is  known  as  phosphorite,  or  rock-phosphate,  and  be- 
ing found  in  large  beds,  it  is  extensively  mined  for  the 
manufacture  of  the  compound  called  superphosphate 
of  lime,  which,  being  soluble  in  water,  can  be  directly 
assimilated  by  plants. 

The  hardness  of  apatite  is  No.  5  on  the  scale;  the 
crystallized  material  can  be  scratched  with  a  knife, 


APATITE  173 


though  not  very  easily;  earthy  masses  may,  however, 
be  quite  soft.  This  degree  of  hardness  affords  an 
easy  means  of  distinguishing  crystals  of  apatite  from 
the  hexagonal  crystals  of  other  minerals  (beryl,  tour- 
maline, quartz),  which  may  somewhat  resemble 
apatite  in  appearance.  The  specific  gravity  (3.2) 
also  serves  for  distinguishing  apatite.  But  in  case 
of  doubt  the  only  sure  way  is  to  test  the  material 
chemically  for  phosphoric  acid  and  calcium. 

Apatite  is  a  mineral  of  wide  distribution.  It  is 
present  in  small  amount  as  minute  crystals  in  all 
igneous  and  crystalline  rocks ;  and  in  veins  traversing 
granite  and  gneiss  it  is  sometimes  found  as  beautiful 
crystals.  It  also  occurs  in  metalliferous  veins,  es- 
pecially in  those  containing  tin-ore.  The  specimen 
represented  in  Fig.  i  is  from  a  tin-mine  at  Greifen- 
stein,  near  Ehrenfriedersdorf,  in  Saxony;  the  bril- 
liant and  transparent  violet  crystals  here  encrust 
crevices  in  an  altered  granitic  rock  known  as  greisen. 
The  extensive  apatite  deposits  of  southern  Norway 
occur  in  connection  with  a  rock  called  gabbro;  while 
those  of  Canada  are  in  crystalline  limestone.  Many 
of  the  rock-phosphates  are  of  recent  formation,  and 
have  been  derived  by  the  action  of  bird  guano  on 
limestone,  often  a  coral  limestone  on  small  islands 
frequented  by  large  flocks  of  birds. 

PYROMORPHITE 

(Plate  23,  Figs.  2  and  3). — This  ore  of  lead  is  an- 
alogous to,  and  isomorphous  with,  apatite,  the  form- 


174        THE    WORLD'S    MINERALS 

ula  being  3Pb3(PO4)2.PbCl2,  or  (PbCl)Pb4(PO4)s 
— that  is,  just  the  same  as  the  formula  of  chlor-apatite, 
with  lead  in  place  of  calcium.  The  crystals  are  also 
extremely  similar,  though  in  pyromorphite,  faces 
other  than  the  hexagonal  prism  and  the  base  are  only 
rarely  present.  In  the  group  of  crystals  shown  in 
Fig.  3  this  characteristic  form  is  plainly  to  be  seen; 
while  in  Fig.  2  the  prisms  are  longer  and  more  closely 
aggregated.  Only  rarely  do  the  crystals  show  any 
degree  of  transparency;  but  they  are  usually  prettily 
colored,  being  wax-yellow,  oraage-y^llpw,  brown 
(Fig.  3),  or  bright  grass-green  (Fig.  2).  On  this 
account  the  mineral  is  variously  known  to  miners  as 
"brown  lead-ore,"  "green  lead-ore,"  or  "variegated 
lead-ore."  Since  it  contains  a  large  amount  of 
lead  (76^  per  cent.),  the  mineral  is  very  heavy 
(sp.  gr.  7.0). 

Pyromorphite  occurs  in  veins  of  lead-ore,  where 
it  has  been  formed  by  the  alteration  of  galena;  the 
phosphate  solutions  producing  the  change  have  per- 
haps in  some  cases  been  derived  from  the  surface 
soil  or  from  guano  deposits.  Good  crystallized  speci- 
mens have  been  found  in  Cornwall;  the  specimen 
represented  in  Fig.  2  is  from  Hofsgrund,  in  Baden, 
and  that  in  Fig.  3  is  from  the  Friedrichssegen  mine, 
near  Ems,  in  Nassau. 

Another  ore  of  lead  very  similar  in  appearance 
to  pyromorphite  is  known  as  mimetite,  this  being  the 
corresponding  arsenic  compound  with  the  formula 
3Pb3  ( AsO4)  2.PbCl2.  The  crystals  are  very  like  those 
of  pyromorphite,  but  are  often  much  curved,  having 


PHOSPHATES,  &c. 


Plate  23. 


1,  Apatite.    2,  3,  Pyromorphite.    4,  Vanadinite. ;  5,\  6,; 


VANADINITE— ERYTHRITE         175 

the  appearance  of  barrels  or  small  balls.  Abundance 
of  crystals  of  this  kind  (known  by  the  name  of 
campylite)  are  to  be  found  among  the  old  mine 
refuse  in  Dry  Gill,  a  deep  valley  on  the  side  of  Car- 
rock  Fell,  in  Cumberland. 

VANADINITE 

(Plate  23,  Fig.  4). — This  is  still  another  mineral 
of  th  .  same  isomorphous  group  of  minerals,  with  the 
analogous  formula  3Pb3(VO4)2.PbCl2,  the  phosphor- 
us of  pyromorphite  being  here  replaced  by  the  rare 
metal  vanadium.  This  mineral  was  formerly  found 
as  brown  crystals  and  small  warty  masses  at  Wan- 
lockhead,  in  Scotland;  and  more  recently  very  bril- 
liant ruby- red  crystals  have  come  from  Arizona  (Fig. 
4).  In  the  picture  there  are  to  be  seen  several  small 
hexagonal  prisms,  with  six-sided  planes  at  their  ends, 
scattered  over  the  surface  of  the  matrix.  Like  pyro- 
morphite and  mimetite,  this  mineral  is  of  secondary 
origin  in  veins  of  lead-ore. 

ERYTHRITE 

(Plate  23,  Figs.  5  and  6). — We  pass  now  to  an- 
other isomorphous  group  of  minerals  known  as  the 
vivianite  group,  for  the  several  members  of  which  the 
formula  may  be  written  generally  as  R"3Q2O8.8H2O, 
where  the  metal  R"  may  be  iron,  cobalt,  nickel,  zinc, 
or  magnesium,  and  Q  stands  for  either  phosphorus 
or  arsenic.  The  type  mineral  (vivianite)  of  this 


176        THE    WORLD'S    MINERALS 

group  will  be  described  presently;  the  one  to 
be  now  considered  is  erythrite,  the  hydrated  arsenate 
of  cobalt,  in  which  the  general  formula  becomes 
Co3As2O8.8H2O. 

The  name  erythrite,  meaning  "red"  in  Greek,  re- 
fers to  the  characteristic  crimson  or  peach-blossom- 
red  color  of  the  mineral.  The  mineral  could  not 
be  confused  with  the  kind  of  sugar  of  the  same  name, 
though  the  identity  in  name  is  certainly  confusing. 
Another  name  often  applied  to  the  mineral  is  cobalt- 
bloom. 

Crystals  of  erythrite  are  small  and  comparatively 
rare.  They  are  monoclinic  and  have  the  form  of 
needles  or  small  blades,  and  are  usually  arranged  in 
radiating  tufts  or  star-like  groups.  They  possess  a 
highly  perfect  cleavage  in  one  direction  parallel  to 
the  plane  of  symmetry,  and  on  this  surface  the  luster 
is  often  beautifully  pearly.  In  Fig.  6  is  shown  a 
number  of  crimson  crystals  scattered  over  the  surface 
of  white  quartz. 

Much  more  frequent  than  crystals  are  earthy  or 
powdery  masses  of  the  same  beautiful  color,  which 
are  to  be  seen  as  encrustations  on  most  specimens  of 
cobalt-ore.  The  mineral,  in  fact,  owes  its  origin  to 
the  weathering  of  cobalt  arsenide  (smaltite,  p.  97). 
Occurring  mixed  in  this  manner  with  cobalt-ore,  it  is 
used  for  the  manufacture  of  cobalt-blue,  so  that  from 
a  mineral  of  a  bright  crimson  color  there  is  obtained 
a  beautiful  sky-blue  product.  The  two  specimens 
represented  in  Figs.  5  and  6  are  both  from  Schnee- 
berg,  in  Saxony. 


ANNABERGITE— WAVELLITE      177 

Analogous  to  cobalt-bloom  we  have  the  corre- 
sponding nickel  compound,  Ni3As2O8.8H2O,  known 
aj  annabergite,  or  nickel-bloom.  This  is  of  a  char- 
acteristic pale  apple-green  color,  and  in  the  same 
way  it  is  produced  by  the  weathering  of  nickel  arsen- 
ide, or  niccolite  (p.  87)  ;  a  small  quantity  of  nickel- 
bloom  is  shown  on  the  specimen  of  niccolite  in  Plate 
6,  Fig.  i. 

WAVELLITE 

(Plate  24,  Fig.  i). — This  is  a  hydrated  basic 
phosphate  of  aluminium,  with  the  formula  aAlPCh. 
A1(OH)3.4^H2O.  Distinct  crystals  are  extremely 
rare,  and  almost  always  this  mineral  is  found  as  hemi- 
spherical or  globular  forms  attached  to  the  surface  of 
slaty  rocks.  When  these  nodules  are  broken  across 
they  show  in  the  interior  a  beautiful  radiated  or  star- 
like  structure,  due  to  the  close  grouping  of  acicular 
crystals  around  a  center.  This  form  and  structure, 
together  with  the  greenish  or  greenish-yellow  color, 
is  indeed  a  very  characteristic  feature  of  the  mineral. 

Wavellite,  though  abundant  at  certain  localities, 
is  not  met  with  at  many  spots.  It  was  first  found 
by  Dr.  W.  Wavel  (after  whom  it  was  named),  at 
the  end  of  the  eighteenth  century,  in  crevices  of  a 
black  slaty  rock  near  Barnstaple,  in  Devonshire.  It 
has  also  been  found  in  Ireland,  and  at  Magnet  Cove, 
Arkansas,  the  specimen  in  Fig.  i  being  from  the 
latter  locality. 


178        THE    WORLD'S    MINERALS 


LAZULITE 

(Plate  24,  Fig.  2). — This  also  is  a  phosphate 
of  aluminium,  but  one  which  contains,  in  addi- 
tion, some  iron  and  magnesium,  the  formula  being 
2AlPO4.(Fe,Mg)  (OH)2.  As  the  name  implies,  this 
mineral  is  of  an  azure-blue  color,  and  so  also  is  the 
mineral,  lazurite.  It  is  therefore  necessary  to  dis- 
tinguish carefully  between  these  two  deep-blue  min- 
erals; lazurite  will  be  described  farther  on  amongst 
the  silicates. 

Lazulite  is  found  as  sharp,  clean-cut  crystals  em- 
bedded in  quartz.  These  crystals  have  very  much 
the  appearance  of  square  tetragonal  pyramids  (Fig. 
2),  but  in  reality  they  are  monoclinic,  with  only  one 
plane  of  symmetry.  It  is  a  comparatively  rare  min- 
eral, found  at  only  a  few  localities.  The  best  speci- 
mens are  from  Werfen,  in  Salzburg,  and  Graves  Mt, 
in  Lincoln  Co.,  Georgia.  The  specimen  figured  is 
from  the  American  locality,  and  shows  the  crystals, 
with  their  characteristic  form  and  color,  embedded 
in  quartz. 

CUPROURANITE 

(Plate  24,  Fig.  3). — Cuprouranite,  or  torbernite, 
is  the  typical  representative  of  an  isomorphous  group 
of  minerals  known  as  the  "uranium  micas,"  so  called 
because  they  contain  uranium  and  form  thin,  platy 
crystals,  with  a  perfect  cleavage  like  the  micas.  They 
are  all  phosphates,  or  arsenates  of  uranium  with 


PHOSPHATES. 


Plate  24. 


1,  Wavellite.    2,  Lazulite.    3,  Torbernite.     4>  Vivianite.    5,  Turquoise. 


CUPROURANITE  179 

copper  or  calcium,  and  water  of  crystallization.  Like 
all  compounds  of  uranium,  they  are  radio-active. 

Cuprouranite  is  the  hydrated  phosphate  of 
uranium  and  copper,  with  the  complex  formula 
CuO.2UO2.P2O5.8H2O.  Its  crystals  belong  to  the 
tetragonal  system,  and  have  the  form  of  thin,  square 
plates,  with  a  perfect  micaceous  cleavage  parallel  to 
the  surface  of  the  plate.  On  this  surface  the  luster 
is  consequently  pearly  in  character.  The  color  is 
always  a  bright  grass-green  (much  more  vivid  than 
represented  in  Fig.  3)  The  crystals  are  quite  soft, 
being  only  slightly  harder  than  gypsum. 

Very  beautiful  crystallized  specimens  have  been 
found  near  the  surface  in  some  of  the  Cornish  mines 
— namely,  near  Calstock,  Grampound  Road,  and 
Redruth.  The  crystals  are  deposited  on  limonite  (as 
in  Fig.  3,  from  Cornwall),  and  they  evidently  have 
been  formed  by  the  alteration  of  the  pitchblende  and 
copper  ores  found  deeper  in  the  same  mines. 

Another  of  the  uranium  micas  is  the  mineral  cal- 
couranite,  or  autunite,  which  has  the  same  chemical 
composition  as  cuprouranite,  except  that  calcium 
takes  the  place  of  copper.  This  crystallizes  in  thin, 
square  plates  of  a  sulphur-yellow  color,  sometimes 
with  a  greenish  tinge.  It  is  found  at  Autun  in  France 
(hence  the  name  autunite},  at  Sabugal  in  Portugal, 
and  some  other  places,  often  as  a  coating  on  the 
surface  of  crevices  in  weathered  granite.  In  Portu- 
gal it  has  quite  recently  been  mined  as  an  ore  of 
radium,  one  gram  of  which  is  extracted  from  600 
tons  of  the  crude  ore — that  is,  one  part.in  600  million. 


180        THE    WORLD'S    MINERALS 


VlVIANITE 

(Plate  24,  Fig.  4) . — It  has  already  been  mentioned 
that  vivianite  is  the  type  member  of  an  isomorphous 
group  of  minerals,  to  which  erythrite  and  annaber- 
gite  also  belong.  In  this  group  vivianite  is  the 
hydrated  phosphate  of  iron  (ferrous  iron),  with 
the  formula  Fe3(PO4)2.8HoO.  Its  crystals  are  mono- 
clinic,  usually  with  a  prismatic  or  blade-like  habit; 
they  have  a  very  perfect  cleavage  parallel  to  the 
plane  of  symmetry,  and  on  this  surface  the  luster  is 
pearly.  The  crystals  are  very  soft,  and  are  easily 
bent  and  distorted.  The  characteristic  color  is  deep 
indigo-blue,  though  sometimes  it  is  greenish-blue. 
When,  however,  a  rock-cavity  containing  crystals  of 
vivianite  is  freshly  broken  open,  the  crystals  are  col- 
orless and  transparent,  but  on  exposure  to  air  and 
light  they  very  quickly  become  blue,  this  change  be- 
ing due  to  a  partial  alteration  of  the  ferrous  iron  to 
ferric  iron.  For  this  reason  the  crystals  to  be  seen 
in  collections  are  always  blue,  and  probably  but  few 
mineralogists  have  had  an  opportunity  of  seeing  the 
colorless  crystals. 

This  specimen  represented  in  Fig.  4  shows  bundles 
of  prismatic  crystals  grown  in  a  cavity  in  massive 
pyrrhotite  which  in  part  has  been  altered  to  limonite. 
Many  fine  specimens  of  this  kind  were  found  years 
ago  in  a  mine  called  Wheal  Jane,  near  Truro,  in 
Cornwall.  Groups  of  larger  crystals  have  been  more 
recently  found  at  Leadville,  in  Colorado. 


VIVIANITE— TURQUOISE  181 

Crystallized  vivianite  is,  however,  of  comparative- 
ly rare  occurrence;  but  in  the  form  of  a  pale-blue 
powder,  known  as  blue  iron-earth,  this  mineral  is  of 
wide  distribution,  being  found  in  practically  all  peat- 
bogs and  deposits  of  bog  iron-ore,  and  especially  on 
bones  and  horns  buried  in  such  bogs.  It  is  also 
sometimes  found  encrusting  the  roots  of  plants  when 
these  are  embedded  in  a  ferruginous  clay. 

TURQUOISE 

(Plate  24,  Fig.  5). — As  a  precious  stone  turquoise 
is  known  to  everybody,  but  probably  few  are  aware 
that  in  chemical  composition  it  is  a  hydrated  phos- 
phate of  aluminium  colored  by  copper  and  iron 
compounds.  The  exact  composition  is,  however, 
rather  doubtful,  but  the  proportions  are  different 
from  those  of  wavellite  (p.  177)  and  the  several 
other  hydrated  phosphates  of  aluminium  that  occur 
as  minerals.  In  turquoise  there  is  usually  about  5 
per  cent,  of  copper  oxide. 

Although  showing  under  the  microscope  a  minute- 
ly crystalline  and  granular  structure,  turquoise  is 
never  found  as  crystals,  but  only  as  nodular  masses, 
or  more  usually  as  a  compact  mineral,  rilling  crevices 
in  rocks,  thus  resembling  opal  in  its  mode  of  occur- 
rence. It  is  an  opaque  mineral,  which,  when  cut, 
takes  a  good  polish,  and  its  use  as  a  precious  stone 
depends  on  its  pure  sky-blue  color  and  soft;  waxy 
luster.  For  use  in  jewelry  it  is  always  cut  with  a 
convex  surface,  and  when  mounted  with  a  surround 


182         THE    WORLD'S    MINERALS 

of  small  diamonds  it  is  very  effective.  Very  clear 
imitations,  so  far  as  color  goes,  are  made  in  glass, 
but  these  do  not  show  quite  the  true  luster  of  the 
genuine  stone,  and  they  have  a  transparent,  glassy 
appearance,  which  is  especially  seen  when  a  frag- 
ment is  broken  off.  The  natural  mineral  is  often  of 
a  greenish  color;  but  such  stones,  though  at  one  time 
in  favor,  are  now  of  very  little  value. 

The  best  turquoises  come  from  near  Meshed,  in 
Persia,  where  they  occur  as  a  filling  in  crevices  in  a 
weathered  rock  of  igneous  origin.  Here  the  mineral 
has  been  mined  for  hundreds  of  years,  and  being  ex- 
ported to  the  west  through  Turkey,  it  came  to  be 
known  as  turquoise.  In  the  Sinai  Peninsula,  tur- 
quoise-mines were  worked  by  the  ancient  Egyptians, 
and  the  stone  was  employed  by  them  as  a  material  for 
carving  scarabs.  Ancient  mines,  worked  in  prehis- 
toric times,  are  situated  in  New  Mexico  and  other 
western  states  of  North  America,  and  at  the  present 
day  very  fine  turquoises  are  obtained  from  this  re- 
gion; the  color  of  some  of  the  American  stones  is, 
however,  rather  liable  to  fade  on  exposure  to  light. 


CHAPTER  XII 

THE  SILICATES 

THE  group  of  silicates  is  by  far  the  largest,  and  at 
the  same  time  the  most  complex,  in  our  chemical 
classification  of  minerals.  Although  a  good  number 
of  species  are  here  described,  yet  there  are  a  great 
many  more  known  to  mineralogists,  several  of  which 
are,  however,  only  of  scientific  interest.  The  abun- 
dance and  complexity  of  silicon  compounds  in  the 
inorganic  world  is  in  a  way  analogous  to  the  vast 
number  of  complex  carbon  compounds  in  the  organic 
world.  It  is  interesting  to  note  that  these  two  non- 
metallic  elements,  carbon  and  silicon,  occupy  ad- 
jacent positions  in  the  same  group  of  the  chemists' 
periodic  classification  of  the  elements ;  in  other  words, 
these  elements  are  closely  related  in  their  chemical 
properties.  The  ultimate  product,  resulting  by  the 
destruction  of  organic  compounds,  is  the  common 
gas,  carbon  dioxide;  and,  similarly,  the  ultimate 
product  produced  by  the  weathering  and  disinte- 
gration of  silicon  compounds  is  silicon  dioxide,  which 
has  been  already  described  as  the  common  minerals 
quartz  and  opal. 

Silicon  dioxide,  or  silica,  when  combined  with 
water,  gives  an  acid  known  as  silicic  acid,  which  can 
be  prepared  in  the  laboratory  as  a  gelatinous  sub- 

183 


184        THE    WORLD'S    MINERALS 

stance.  The  proportions  of  water  and  silica  may  vary : 
thus  in  meta-silicic  acid  (H2O  +  SiO2  =  H2SiO3) 
we  have  equal  molecular  proportions — one  molecule 
of  water  combined  with  one  molecule  of  silica ;  while 
in  ortho-silicic  acid  (2H2O  +  SiO2  =  H4SiO4)  they 
are  in  the  ratio  of  two  to  one.  Several  other  hypo- 
thetical silicic  acids  have  to  be  assumed  to  explain 
the  chemical  composition  of  various  silicates  which 
occur  in  nature  as  minerals.  The  formula  used  to 
express  the  chemical  composition  of  these  minerals 
are  often  very  complex,  and  in  some  cases  not  yet 
completely  determined  by  mineral  chemists.  It 
would,  therefore,  be  out  of  place  in  this  book  to  enter 
into  much  detail  in  this  direction,  and  we  must  con- 
tent ourselves  by  stating  the  principal  elements  that 
are  present  in  the  more  complex  minerals. 

The  simple  silicates  of  the  alkali  metals — for  ex- 
ample, sodium  silicate  and  potassium  silicate  (known 
as  water-glass] —  are  soluble  in  water.  But  all  the 
naturally  occurring  silicates  are  insoluble  in  water, 
and  most  of  them  are  unattacked  by  acids  (except 
hydrofluoric  acid)  ;  further,  they  can  be  fused  only 
with  great  difficulty.  On  the  other  hand,  the  com- 
pounds of  carbon  are  more  readily  dealt  with  and 
can  be  prepared  in  the  laboratory;  so  that  by  a  va- 
riety of  reactions  and  replacements  the  organic  chem- 
ist is  able  to  draw  conclusions  as  to  the  chemical  con- 
stitution of  such  compounds.  The  mineral  chemist 
is,  however,  placed  at  a  disadvantage  by  reason  of 
the  intractable  nature  of  the  material  he  has  to  deal 
with.  For  instance,  the  usual  way  of  decomposing 


THE    SILICATES  185 

a  silicate  for  analysis  is  to  fuse  the  finely  powdered 
mineral  with  sodium  carbonate  in  a  platinum  crucible 
over  the  intense  flame  of  a  blowpipe.  Mineral  sili- 
cates can  be  prepared  artificially  only  with  great  dif- 
ficulty ac  very  high  temperatures,  and  even  when  the 
experiments  extend  over  a  period  of  several  days  only 
crystals  of  microscopic  dimensions  are  obtained.  The 
element  of  time  is  here  a  matter  of  consequence;  and 
in  nature's  laboratory,  where  high  temperatures  and 
pressures  are  available  for  long  periods,  very  finely 
crystallized  products  result — indeed,  amongst  the 
silicates  we  find  some  of  the  most  beautiful  of  crystal- 
lizations. 

It  is  not  surprising,  therefore,  that  but  little  knowl- 
edge has  been  acquired  respecting  the  chemical  con- 
stitution of  naturally  occurring  silicates.  And  for 
this  reason  no  really  satisfactory  classification  of  the 
whole  group  has  yet  been  devised,  different  authors 
using  different  systems  of  classification.  Neverthe- 
less, certain  closely  related  species  fall  naturally 
together  into  a  number  of  isomorphous  groups.  In 
the  following  pages  some  of  these  groups  will  be 
considered,  while  some  other  species  must  be  dealt 
with  singly  in  no  particular  order. 

The  silicates  are  of  prime  importance  as  rock- 
forming  minerals,  since  they  form  the  bulk  of  the 
rocks  of  the  earth's  crust.  All  the  rocks  of  igneous 
and  metamorphic  origin  are  composed  almost  en- 
tirely of  silicates,  sometimes  with  the  addition  of 
quartz;  and  it  is  only  in  certain  rocks  of  sedimen- 
tary origin,  such  as  limestone  and  the  pure  quartz- 


186        THE    WORLD'S    MINERALS 

sandstones,  that  they  are  absent.  The  silicates 
are  also  of  importance  from  other  points  of  view. 
Several  species  are  used  as  precious  stones  or  for 
ornamental  purposes;  while  others  have  important 
technical  applications. 

THE  FELSPAR  GROUP 

This  is  a  very  important  group  of  rock-forming 
minerals,  which  are  analogous  in  chemical  compo- 
sition and  similar  in  crystalline  form;  that  is,  they 
form  an  isomorphous  group.  They  are  silicates  of 
aluminium,  together  with  either  potassium,  sodium, 
or  calcium ;  so  that,  chemically,  we  have  three  kinds 
of  felspar — namely,  potash-felspar,  soda-felspar,  and 
lime-felspar.  Again,  although  very  similar  to  one 
another  in  the  form  of  their  crystals,  yet  these  belong 
to  two  different  systems,  the  monoclinic  and  the 
triclinic.  We  may,  therefore,  divide  the  felspars  into 
monoclinic  felspars  and  triclinic  felspars. 

A  character  of  special  importance,  and  one  com- 
mon to  all  kinds  of  felspar,  is  that  of  cleavage.  The 
crystals  possess  a  perfect  cleavage  parallel  to  the  basal 
pinacoid,  this  being  called  the  basal  cleavage;  and  a 
second,  rather  less  perfect  cleavage,  parallel  to  the 
brachy-pinacoid,  called  the  brachy-pinacoidal  cleav- 
age. Now  in  monoclinic  felspar  the  second  cleavage 
is  parallel  to  the  single  plane  of  symmetry,  which  is 
perpendicular  to  the  basal  plane.  The  two  directions 
of  cleavage  are  here,  therefore,  at  right  angles  to  one 
another,  and  this  kind  of  felspar  is  consequently 


THE    FELSPAR    GROUP  187 

called  orthoclase,  meaning,  in  Greek,  "splitting  at 
right  angles."  In  the  triclinic  felspars,  on  the  other 
hand,  the  two  cleavages  are  not  quite  at  right  angles, 
but  inclined  at  an  angle  varying  in  the  different  spe- 
cies from  86°  24'  (albite)  to  85°  50'  (anorthite) ; 
these  are  therefore  referred  to  collectively  as  plagio- 
clase,  which  means  "splitting  obliquely."  In  addi- 
tion to  these  we  have  another  kind  of  triclinic  felspar 
known  as  microcline,  so  called  because  the  angle  be- 
tween the  cleavages  differs  only  very  slightly  from  a 
right  angle,  being  about  89°  30'. 

The  various  kinds  of  felspar  may  now  be  tabulated 
as  follows : 

Chemical  System  of 

Formula  Crystallization 

Orthoclase,  or  potash-felspar. .     KAlSi3O8  Monoclinic 

Microcline,  or  potash- felspar. .     KAlSi3O8  Triclinic 

Albite,  or  soda-felspar NaAlSi3O'8         Triclinic 

Anorthite,  or  lime-felspar CaAlzSiaOs        Triclinic 

By  the  mixing  together  of  albite  and  anorthite  in 
indefinite  proportions,  we  have  other  members  of  the 
plagioclase  series,  which  are  known  as  oligoclase, 
andesine,  and  labradorite;  these  are  intermediate  be- 
tween albite  and  anorthite,  not  only  in  chemical 
composition,  but  in  all  their  properties. 

Twinned  crystals  are  of  very  frequent  occurrence 
in  all  the  felspars,  and  several  types  of  twinning  are 
known.  The  plagioclase  felspars  are  invariably 
twinned  according  to  the  "albite  law,"  one  portion 
of  a  twinned  crystal  being  a  reflection  of  the  other 
portion  across  a  plane — the  twin-plane — parallel  to 
the  brachy-pinacoid.  In  one  and  the  same  crystal 


188        THE    WORLD'S    MINERALS 

this  twinning  is  repeated  many  times,  so  that  the 
whole  crystal  consists  of  a  pile  of  twin-lamellae.  This 
repeated  lamellar  twinning  gives  rise  to  a  very  char- 
acteristic appearance,  to  be  seen  when  basal  cleavage 
flakes,  or  thin  sections  of  plagioclase  crystals,  are  ex- 
amined in  polarized  light  under  the  microscope. 
Further,  it  gives  rise  to  a  system  of  fine  lines  on  the 
basal  plane  and  also  on  the  basal  cleavage;  these  lines 
or  striations  being  parallel  to  the  edge  between  the 
basal  plane  and  the  brachy-pinacoid.  On  this  account 
the  plagioclase  felspars  are  known  also  as  "striated 
felspars" ;  and  in  hand-specimens  they  can  always  be 
recognized  by  this  character. 

In  addition  to  similarity  of  crystalline  form  and 
cleavage,  the  felspars  all  possess  the  same  degree  of 
hardness  (No.  6  on  the  scale).  They  may  be  color- 
less and  transparent,  but  more  often  are  white  or 
pinkish  and  opaque.  They  are  not  heavy,  the  specific 
v  gravity  ranging  with  the  chemical  composition  from 
2.55  (orthoclase)  102.75  (anorthite).  If,  therefore, 
we  have  a  white,  opaque  mineral  with  two  good 
cleavages,  nearly  or  quite  at  right  angles,  which  is  not 
heavy,  and  can  only  just  be  scratched  with  a  knife, 
we  may  be  pretty  sure  that  the  mineral  in  question  is 
felspar. 

There  are  extremely  few  kinds  of  igneous  rocks 
and  gneisses  which  do  not  contain  a  large  proportion 
of  one  or  other  of  the  felspars;  so  that  these  minerals 
are  abundant  and  of  wide  distribution.  By  the 
weathering  and  breaking  down  of  these  rocks  the 
felspars  are  decomposed,  their  alkalis  being  assim- 


FELSPAR:     ORTHOCLASE  189 

ilated  by  plants  or  carried  away  in  solution;  while 
the  alumina  and  silica  combine  with  water  to  form 
hydrated  aluminium  silicate.  The  latter,  together 
with  other  mineral  particles,  may  be  removed  by  run- 
ning water  and  deposited  as  beds  of  clay,  such  as  is 
used  for  the  making  of  bricks  and  pottery.  A  purer 
form  of  white  clay,  known  as  china-clay,  or  kaolin, 
also  results  from  the  decomposition  under  special  con- 
ditions of  the  felspar  of  granite.  We  thus  see  that, 
indirectly  at  least,  the  felspars  are  of  considerable 
economic  importance. 

ORTHOCLASE 

(Plate  25,  Figs,  i  and  2). — This  is  potash-felspar, 
crystallizing  in  the  monoclinic  system.  It  is  the  most 
abundant  of  the  felspars,  and  the  one  most  commonly 
found  as  large,  well-formed  crystals.  For  this  rea- 
son it  is  sometimes  called  "common  felspar."  It  is 
an  important  constituent  of  all  granites  and  syenites, 
of  the  lavas  known  as  rhyolite  and  trachyte,  and  of 
several  other  kinds  of  igneous  rocks,  more  especially 
those  which  also  contain  some  quartz.  It  is  also 
abundant  in  gneisses;  and  sometimes  it  is  present  in 
the  sandstones  and  grits  which  have  been  formed  of 
the  debris  of  such  rocks.  The  pegmatite-veins  as- 
sociated with  granitic  masses  consist  largely  of  coarse- 
ly crystallized  orthoclase.  Cavities  in  granite,  and 
more  especially  in  pegmatite,  are  often  lined  with 
well-formed  crystals  of  orthoclase,  together  with 
crystals  of  topaz,  tourmaline,  and  other  minerals. 


190        THE    WORLD'S    MINERALS 

In  Plate  25,  Fig.  i,  is  shown  a  group  of  orthoclase 
crystals,  and  in  Fig.  2  a  single  crystal,  taken  from  a 
cavity  in  the  granite  at  Alabashka,  near  Mursinka, 
in  the  Urals.  In  both  specimens  the  reddish  felspar 
is  associated  with  smoky-quartz;  and  in  Fig.  2  there 
is  a  regular  interpenetration  of  the  two  minerals.  The 
crystals  here  represented  are  bounded  by  a  rhombic 
prism,  the  clino-pinacoid  (on  the  right  in  Fig.  2), 
and  the  basal  plane  and  an  ortho-pinacoid  at  the  top. 

The  crystals  from  granite  have  frequently  a  dull, 
chalky  or  stony  appearance;  but  certain  crystals  have 
quite  the  appearance  of  glass,  and  are  thus  known  as 
glassy  felspar,  or  ice-spar,  or  more  commonly  as  sani- 
dine.  Crystals  of  sanidine  are  found  embedded  in 
the  trachytic  lavas  of  the  Rhenish  district.  Clear 
crystals  of  the  variety  called  adularia  are  found  lining 
crevices  in  the  gneissic  rocks  of  the  Swiss  Alps.  Still 
another  variety  of  orthoclase  is  that  known  as  moon- 
stone, so  called  because  of  the  moon-like  reflection 
or  opalescence,  seen  on  its  surface.  Such  stones  are 
found  plentifully  in  Ceylon,  and  are  cut  and  polished, 
with  a  rounded  surface,  for  use  in  jewelry.  Rarely 
to  be  seen  in  orthoclase  are  the  brilliant  blue,  green, 
or  yellow  sheens,  or  colored  reflections,  of  the  same 
nature  as  those  characteristic  of  labradorite; 
material  of  this  kind  forms  a  large  part  of  the  augite- 
syenite  of  Laurvik  and  Fredriksvarn,  in  southern 
Norway,  a  rock  which,  when  cut  into  slabs  and  pol- 
ished, is  extensively  used  for  ornamental  purposes. 

Large  quantities  of  orthoclase  are  quarried  from 
the  pegmatite-veins  of  southern  Norway,  the  material 


SILICATES  (Felspar  group). 


Plate  25. 


1,  2,  Orthoclase.    3,  Labradorite.    4,  Microcline.    5,  Anorthite. 


FELSPAR:     MICROCLINE  191 

being  used  in  the  manufacture  of  porcelain,  more 
especially  for  the  production  of  the  glaze. 

MICROCLINE 

(Plate  25,  Fig.  4). — This  also  is  potash-felspar, 
and  is  identical,  chemically,  with  orthoclase.  Crystal- 
lographically,  however,  it  is  triclinic,  while  ortho- 
clase is  monoclinic;  so  that  these  two  species  are  to 
be  considered  as  dimorphic  forms  of  potash-felspar. 
In  their  external  appearance,  however,  crystals  of 
microcline  cannot  be  distinguished  from  crystals  of 
orthoclase,  and  indeed  the  only  method  of  distin- 
guishing the  two  is  by  means  of  their  optical  char- 
acters as  observed  under  the  polarizing  microscope. 
Basal  cleavage  flakes  or  thin  sections  of  microcline 
show  in  polarized  light  a  very  characteristic  cross- 
hatched  structure,  due  to  repeated  lamellar  twinning 
in  two  directions.  Orthoclase  and  microcline  are  thus 
only  to  be  distinguished  by  special  optical  tests,  and 
the  difference  between  them  seems  to  be  a  matter  of 
only  slight  importance,  and  especially  so  when  we 
bear  in  mind  that  they  occur  in  nature  under  ex- 
actly the  same  conditions  as  constituents  of  granite 
and  pegmatite.  Indeed,  many  mineralogists  assert 
that  they  are  in  reality  identical,  the  cross-hatching 
being  on  so  minute  a  scale  in  orthoclase  that  it  is  not 
visible  under  the  higher  powers  of  the  polarizing 
microscope.  This  ultra-microscopic  twinning  would 
account  for  the  observed  differences  in  the  optical 
characters  of  orthoclase  and  microcline. 


192        THE    WORLD'S    MINERALS 

A  beautiful  green  variety  of  microcline  deserves 
special  mention.  This  is  known  as  amazon-stone 
(Fig.  4)  ;  it  is  found  as  large,  well-shaped  crystals 
in  cavities  in  granite  at  a  few  places,  the  specimen 
represented  in  the  picture  being  from  Pike's  Peak, 
in  Colorado.  Here  we  have  a  parallel  intergrowth 
of  two  crystals,  each  bounded  by  the  rhombic  prism, 
clino-pinacoid,  and  at  the  top  by  the  basal  plane  and 
an  ortho-pinacoid.  Amazon-stone  is  sometimes  cut 
and  polished  for  the  construction  of  various  small 
ornamental  objects. 

ALBITE 

Mention  has  already  been  made  of  the  plagioclase 
felspars,  which  form  a  continuous  isomorphous  se- 
ries, ranging  from  albite  to  anorthite.  The  soda- 
felspar  called  albite,  which  forms  one  end  of  this 
series,  is  of  importance  as  a  rock-making  mineral, 
and  it  is  also  of  interest  to  collectors  as  being  found 
in  pretty  groups  of  white  crystals.  Such  crystals  are 
found,  together  with  clear  crystals  of  quartz  (rock- 
crystal),  attached  to  the  walls  of  crevices  in  the 
gneissic  rocks  of  the  Alps  and  in  the  slates  at  Tintagel, 
in  Cornwall.  The  name  albite  refers  to  the  white 
color  of  the  crystals. 

OLIGOCLASE 

Oligoclase  is  an  intermediate  member  of  the  pla- 
gioclase series  of  felspars,  consisting  of  a  mixture  of 
three  to  six  parts  of  albite  with  one  part  of  anor- 


FELSPAR:     LABRADORITE          193 

thite.  It  is  to  this  species  that  the  beautiful  variety 
known  as  sun-stone,  or  avanturine  felspar,  belongs. 
This  shows  on  its  surface  a  very  pretty  metallic 
spangled  reflection,  due  to  the  enclosure  in  the  stone 
of  numerous  small  scales  of  haematite.  The  best 
specimens — such  as  are  cut  and  polished  for  use 
in  jewrelry — are  from  Tvedstrand,  in  the  south  of 
Norway. 

LABRADORITE 

(Plate  25,  Fig.  3). — Labradorite,  or  labrador-spar, 
is  another  intermediate  member  of  the  plagioclase 
group  of  felspars,  and  consists  of  a  mixture  of  one 
part  of  albite  with  three  to  six  parts  of  anorthite.  It 
is  an  important  constituent  of  the  more  basic  igneous 
rocks,  such  as  basalt  and  gabbro.  In  these  rocks  it 
is  present  as  smaller  or  larger  embedded  crystalline 
grains;  crystals  bounded  by  faces  being  almost  un- 
known. On  the  coast  of  Labrador  it  forms,  with 
hypersthene  (Plate  27,  Fig.  5),  a  rock  called  norite, 
which  is  here  so  very  coarsely  grained  that  the  mas- 
sive pieces  of  labradorite  measure  as  much  as  a  foot 
across.  In  this  region  pebbles  and  boulders  of  lab- 
radorite are  widely  scattered  over  the  surface,  the 
material  having  been  dislodged  from  the  solid  rock 
by  the  action  of  ice.  These  stones  are  of  a  dull, 
greyish-black  color  and  opaque;  but  when  a  wet  peb- 
ble is  held  in  the  hand  and  turned  slowly  round,  at 
a  certain  position,  it  suddenly  flashes  with  a  bril- 
liantly colored  metallic  reflection.  The  color  is  an 
intense  red,  yellow,  green,  or  blue,  resembling  the 


194        THE    WORLD'S    MINERALS 

brilliant  metallic  colors  of  a  peacock's  feather  or 
the  wings  of  some  tropical  butterflies.  When  the 
stone  is  turned  away  from  this  position,  however 
slightly,  the  color  totally  disappears.  This  curious 
and  pretty  effect  must  have  been  noticed  long  ago  by 
the  Esquimaux;  but  it  was  not  until  1775  that  speci- 
mens reached  Europe,  when  they  were  brought  by  the 
Moravian  missionaries. 

The  brilliancy  of  the  reflection  is  much  increased 
when  the  stone  is  cut  with  a  flat  surface  and  polished ; 
but  even  then  it  is  only  in  one  definite  position  of 
the  stone  relative  to  the  eye  of  the  observer  that  the 
color  is  seen. 

These  colored  reflections  of  labradorite  are  due  to 
the  enclosure  in  the  felspar  substance  of  vast  numbers 
of  microscopic  plates  of  other  minerals  all  ar- 
ranged parallel  to  a  certain  crystallographic  direc- 
tion. The  substance  itself  is  not  colored,  and  the 
colors  are  produced  by  the  splitting  up  and  reflection 
of  a  portion  of  the  white  light  when  its  rays  strike 
the  parallel  series  of  minute  plates  at  a  particular 
inclination. 

This  special  optical  effect  is  thus  quite  an  acci- 
dental character  of  labradorite,  and  it  is,  in  fact, 
shown  only  by  the  labradorite  from  certain  localities, 
more  particularly  from  Labrador  (Fig.  3),  and  less 
vividly  in  some  specimens  from  Russia.  Labradorite 
is  a  common  constituent  of  certain  igneous  rocks  in 
the  British  Isles;  but  none  of  this  material  displays 
the  beautiful  interference-colors. 


FELSPAR:     ANORTHITE  195 


ANORTHITE 

(Plate  25,  Fig.  5). — Anorthite,  or  lime-felspar, 
finds  its  place  at  the  other  end  of  the  plagioclase 
series.  The  name  refers  to  the  fact  that  none  of  the 
angles  on  the  crystals  are  right  angles,  the  crystals 
belonging  to  the  triclinic  or  anorthic  system.  It  is 
an  important  constituent  of  some  of  the  more  basic 
rocks  of  igneous  origin,  such  as  basalt  and  gabbro, 
and  it  has  also  been  detected  in  some  meteoric  stones. 
In  igneous  rocks  it  is  present  only  as  dull,  irregular 
grains  or  imperfect  crystals.  Beautifully  clear  and 
glassy  crystals,  with  a  profusion  of  small,  brilliant 
faces,  are,  however,  found  in  ejected  blocks  of  meta- 
morphic  limestone  amongst  the  lavas  of  Monte  Som- 
ma,  the  ancient  portion  of  Mount  Vesuvius.  These 
blocks  had  been  ejected  long  before  the  sudden  and 
disastrous  eruption  of  79  A.  D.  which  built  up  the 
present  cone.  Good  but  opaque  crystals,  of  a  pinkish 
color,  are  also  found  in  a  metamorphic  limestone  on 
the  Pesmeda-Alp  in  Fleims  valley,  in  southern  Tyrol ; 
a  very  fine  crystal  from  this  locality  is  represented 
in  Fig.  5. 

THE  AMPHIBOLE  GROUP 

This  is  another  group  of  isomorphous  minerals, 
which  are  of  importance  as  constituents  of  silicate 
rocks,  both  those  of  igneous  and  of  metamorphic 
origin.  Their  chemical  composition,  though  varying 
between  wide  limits,  can  always  be  expressed  by 


196        THE    WORLD'S    MINERALS 

the  general  formula  R"SiO3 — that  is,  as  salts  of  meta- 
silicic  acid  (see  p.  184).  In  this  formula,  R"  usually 
represents  calcium,  magnesium,  and  ferrous  iron,  but 
sometimes  manganese  may  be  present,  or  aluminium 
or  ferric  iron  in  conjunction  with  sodium.  The 
actual  formulae  may  thus  be  extremely  complex. 

All  the  members  of  the  group  (with  a  few  ex- 
ceptions, which  need  not  be  mentioned  in  this  book) 
crystallize  in  the  monoclinic  system,  and  the  crystals 
are  usually  prismatic  in  habit.  This  prismatic  habit 
is  often  so  pronounced  that  the  crystals  have  the  form 
of  fine  fibers,  needles,  or  even  hairs.  Of  importance 
is  the  presence  of  two  perfect  cleavages  parallel  to 
the  faces  of  the  rhombic  prism,  the  angle  of  which 
is  about  124°. 

HORNBLENDE 

(Plate  26,  Fig.  i). — Common  hornblende  occurs 
as  an  essential  constituent  of  a  variety  of  rocks — for 
example,  granite  (hornblende-granite),  syenite,  di- 
orite;  certain  volcanic  lavas,  such  as  some  basalts  and 
andesites;  gneisses,  schists  (hornblende-schist),  and 
metamorphic  limestones. 

Usually  it  forms  bladed  masses  embedded  in  the 
rock,  on  the  broken  surfaces  of  which  the  bright 
prismatic  cleavages  of  the  hornblende  are  conspic- 
uous. In  color  it  is  usually  dark  green  or  black,  but 
sometimes  brown.  The  most  perfectly  developed 
crystals  are  those  found  in  limestones  which  have 
been  baked  by  close  contact  with  a  molten  mass  of 
igneous  rock.  Good  crystals  are  also  found,  together 


HORNBLENDE— TREMOLITE       197 

with  other  crystallized  silicates,  in  the  magnetite- 
mines  of  Arendal,  in  Norway  (Fig.  i). 

The  crystal  here  represented  has  the  form  of  a  six- 
sided  prism,  terminated  by  three  rhomb-shaped  faces ; 
and  it  has  very  much  the  appearance  of  a  hexagonal 
prism  with  a  rhombohedral  termination  (compare, 
for  example,  the  rhombohedral  crystal  of  tourmaline 
shown  in  Plate  33,  Fig.  2).  The  six-sided  prism 
is  here,  however,  a  combination  of  the  four  faces  of 
a  rhombic  prism,  with  the  angle  of  124°  (i.e.  not  very 
far  from  120°),  and  two  faces  of  the  clino-pinacoid 
parallel  to  the  clino-axis  or  plane  of  symmetry  (at 
the  sides  of  the  figure) .  The  two  rhomb-shaped  faces 
at  the  top  are  pyramid  faces,  and  the  lower  one  be- 
longs to  an  ortho-pinacoid  (perpendicular  to  the 
plane  of  symmetry).  Such  a  crystal  can  be  cleaved 
parallel  to  the  faces  of  the  rhombic  prism  only,  and 
not  parallel  to  the  faces  of  the  clino-pinacoid;  where- 
as if  the  form  were  truly  hexagonal  it  could  be 
cleaved  equally  well  parallel  to  all  the  prism  faces. 

TREMOLITE 

Tremolite,  or  white  amphibole,  is,  chemically,  the 
simplest  member  of  the  group,  its  composition  being 
expressed  by  the  formula  CaMg3(SiO3)4.  It  is  found 
as  aggregates  of  bladed  crystals  embedded  in  white 
crystalline  limestone,  or  marble,  at  Tremola,  in 
Switzerland,  and  in  several  other  regions  where 
metamorphic  rocks  occur. 


198         THE    WORLD'S    MINERALS 


ACTINOLITE 

(Plate  26,  Fig.  2). — Here  we  have  some  of  the 
magnesium  in  the  formula  just  given  for  tremolite 
replaced  by  a  small  amount  of  ferrous  iron,  and  it  is 
to  this  constituent  that  the  mineral  owes  its  charac- 
teristic bright  green  color.  As  bladed  or  needle-like 
crystals,  it  is  a  common  constituent  of  many  schistose 
rocks,  and  in  some  cases  it  makes  up  the  bulk  of  such 
rocks.  In  Fig.  2,  representing  a  specimen  from  the 
Zillerthal,  Tyrol,  dark  green  blades  of  actinolite 
penetrate  in  all  directions  a  pale  green  talc-schist. 
The  name  actinolite  is  a  Greek  form  of  the  old  Ger- 
man name  Strahlstein,  meaning  "ray-stone,"  in  allu- 
sion to  the  ray-like  form  of  the  crystals. 

ASBESTOS 

Asbestos  is  identical,  both  chemically  and  crystal- 
lographically,  with  tremolite  and  actinolite,  the 
whiter  varieties,  with  little  or  no  iron,  being  near  to 
tremolite.  Here  the  crystals,  instead  of  being  bladed 
or  acicular,  are  thread-like,  being  enormously  elon- 
gated prismatic  crystals,  so  fine  that  they  are  flexible. 
The  difference  is  thus  merely  one  of  texture.  The 
fibres  sometimes  grow  like  hairs  from  a  rock  surface, 
or  they  may  form  bundles  in  the  crevices  of  horn- 
blende-schists and  other  rocks  of  metamorphic  origin. 

This  mineral  has  some  curious  properties  and  uses. 
It  may  be  combed,  spun,  and  woven  into  cloth,  or 


SILICATES  (Amphibolc  group). 


Plate  26. 


1,  Hornblende.    2,  Actinolite.    3,  Crocidolite.    4,  Nephrite. 


ASBESTOS— NEPHRITE  199 

made  into  string  and  lamp-wicks.  Being  unaffected 
by  fire  (the  name  asbestos  means,  in  Greek,  "un- 
quenchable"), and  at  the  same  time  a  non-conductor 
of  heat,  it  is  used  in  a  variety  of  forms  as  a  fire-proof 
and  insulating  material — for  example,  theatre  cur- 
tains, firemen's  clothes,  linings  for  iron  safes,  packing 
for  the  pistons  of  steam-engines,  coverings  for  steam 
and  hot-water  pipes  and  cold-storage  plants,  etc. 
Napkins  made  of  asbestos  may  be  cleansed  by  placing 
them  on  a  bright  fire. 

The  best  qualities  of  asbestos,  with  fibers  up  to  six 
feet  in  length,  come  from  Lombardy  and  Piedmont, 
in  the  north  of  Italy.  The  greater  part  of  the  asbestos 
used  commercially  is  not  the  amphibole-asbestos  here 
described,  but  a  fibrous  variety  of  the  mineral  ser- 
pentine, known  as  chrysotlle  or  serpentine-asbestos, 
to  be  mentioned  farther  on.  Still  another  finely 
fibrous,  asbestos-like  mineral  is  the  blue  asbestos,  or 
crocidolite,  to  be  mentioned  presently. 

NEPHRITE 

(Plate  26,  Fig.  4). — This,  again,  is  a  variety  of 
amphibole,  which,  like  asbestos,  is  identical  in  its 
essential  characters  with  tremolite  and  actinolite. 
The  structure  is  also  finely  fibrous ;  but  here  the  fibers 
are  short,  and  so  closely  compacted  and  matted  to- 
gether that  they  are  only  recognizable  when  thin 
sections  of  the  mineral  are  examined  under  the  micro- 
scope. This  peculiarity  of  structure  gives  to  the  stone 
an  extraordinary  degree  of  toughness,  so  much  so  that 


200        THE    WORLD'S    MINERALS 

it  is  extremely  difficult  to  break  a  pebble  or  boulder 
of  nephrite  by  blows  from  a  hammer.  The  fracture 
is  splintery;  and  the  hardness,  as  determined  by 
scratching,  is  not  more  than  6  on  the  scale.  Being 
very  compact  and  fairly  hard,  the  mineral  takes  a 
very  good  polish;  and  on  the  polished  surface  the 
luster  is  somewhat  greasy  in  character.  The  color 
is  usually  green  of  various  shades,  depending  on  the 
amount  of  iron  present;  but  it  may  sometimes  be 
white  if  iron  is  absent. 

In  general  character  and  appearance  nephrite  is 
absolutely  indistinguishable  from  the  mineral  ]adeite, 
a  member  of  the  pyroxene  group  (p.  202).  Both 
these  minerals  are  included  under  the  general  term 
jade,  and  both  are  employed  for  the  elaborate  carv- 
ings so  highly  prized  by  the  Chinese.  They  differ 
widely,  however,  in  their  essential  characters,  notably 
in  their  chemical  composition,  nephrite  being  a  sili- 
cate of  calcium  and  magnesium,  with  often  a  little 
iron;  while  jadeite  is  a  silicate  of  sodium  and  alu- 
minium. The  easiest  method  of  distinguishing  the 
two  kinds  is  by  their  specific  gravity,  which  may  be 
determined  by  weighing  the  carved  ornaments  in  air 
and  in  water.  The  specific  gravity  of  nephrite  is 
3.0  (or  up  to  3.1,  with  darker  green  varieties),  while 
that  of  jadeite  is  3.33.  Nephrite,  therefore,  floats 
in  methylene  iodide;  while  jadeite  neither  sinks  nor 
floats,  but  remains  suspended  in  the  liquid. 

Prehistoric  celts  of  nephrite  have  been  found  in 
many  parts  of  Europe,  particularly  in  the  ancient 
lake-dwellings  in  Switzerland.  Axes  and  other 


NEPHRITE— CROCIDOLITE         201 

weapons  fashioned  in  nephrite,  as  well  as  curious 
idols,  with  eyes  of  inlaid  mother-of-pearl,  were  found 
in  the  possession  of  the  Maoris  when  New  Zealand 
was  discovered  by  Tasman  in  1642.  In  China  and 
India  the  use  of  this  mineral  for  carvings  dates  back 
many  centuries,  the  nephrite  quarries  in  the  Kuen- 
Lun  Mountains  having  been  worked  by  the  Chinese 
for  two  thousand  years.  Several  other  localities  pro- 
ducing nephrite  are  known  in  central  Asia,  but  usual- 
ly the  material  is  found  as  water-worn  pebbles  in  the 
beds  of  rivers  and  streams.  Much  of  the  material 
now  cut  in  Europe  for  pendants  and  other  small 
personal  ornaments  comes  from  New  Zealand,  and  is 
generally  known  as  New  Zealand  greenstone.  This  is 
of  a  darker  shade  of  green  (Fig.  4)  than  most  of  the 
Asiatic  nephrite. 

The  name  nephrite  is  from  the  Latin  Lapis  neph- 
riticus,  or  "kidney-stone,"  a  name  so  given  because 
this  stone  was  worn  by  the  ancients  as  a  charm  in  the 
belief  that  it  prevented  kidney  disease. 

CROCIDOLITE 

(Plate  26,  Fig.  3). — This  is  a  dark  blue,  fibrous 
variety  of  amphibole,  differing  considerably  in  chem- 
ical composition  from  the  preceding  varieties.  It  is 
a  silicate  of  sodium  and  iron,  and  has  long  been 
familiar  through  specimens  from  the  Asbestos  Moun- 
tains, near  the  Orange  River,  in  South  Africa,  where 
it  occurs  as  veins  in  jasper-schists.  The  veins  are 
filled  from  wall  to  wall  by  a  closely  compacted  mass 


202         THE  WORLD'S   MINERALS 

of  very  fine  fibers,  which,  when  the  stone  is  rubbed, 
separate  out  into  a  soft,  woolly  mass.  It  was  this 
character  that  suggested  the  name  crocidolite,  from 
the  Greek  name  for  wool.  The  mineral  is  mined  in 
the  Asbestos  Mountains,  and  is  put  to  the  same  com- 
mercial uses  as  the  other  kinds  of  asbestos. 

In  part  of  the  specimen  represented  in  the  picture 
(Fig.  3)  the  characteristic  blue  color  of  the  mineral 
gives  place  to  a  golden-yellow.  This  change  in  color 
is  the  result  of  a  chemical  alteration  of  the  mineral, 
the  iron  it  contains  as  ferrous  silicate  being  oxidized 
and  hydrated  with  the  formation  of  limonite.  At  the 
same  time  this  alteration  is  accompanied  by  the  sepa- 
ration of  free  silica  as  quartz,  which  assumes  the 
fibrous  structure  of  the  original  mineral.  We  then 
have  a  pseudomorph  of  quartz  and  limonite  after 
crocidolite;  and  this  is  the  manner  in  which  the  beau- 
tiful gem-stone  called  tiger-eye  (p.  116)  has  had  its 
origin.  Unfortunately  the  name  crocidolite  is  incor- 
rectly applied  in  the  trade  to  this  pseudo-crocidolite. 

THE  PYROXENE  GROUP 

This  is  another  important  group  of  rock-forming 
minerals,  the  members  of  which  are  in  many  respects 
very  much  like  those  of  the  amphibole  group. 
They  have  the  same  type  of  chemical  formula, 
R"SiO3,  and  are  also  very  much  of  the  same  type  in 
crystallization;  but  with  this  important  difference, 
that  the  angle  between  the  prismatic  cleavages  is  here 
nearly  93°,  instead  of  124°  as  in  the  amphiboles. 


THE    PYROXENE    GROUP          203 

There  are  also  important  differences  in  the  optical 
characters  of  the  crystals. 

This  rhombic  prism,  with  angles  approximating  to 
right  angles,  is  usually  combined  with  two  pinacoids, 
respectively  parallel  and  perpendicular  to  the  single 
plane  of  symmetry;  so  that  we  have  a  nearly  regular 
eight-sided  prism  (Plate  27,  Figs.  1-3),  much  like 
the  eight-sided  tetragonal  prism  of  zircon  (shown 
in  Plate  14,  Fig.  4).  The  crystals  of  the  pyroxenes 
shown  in  Plate  27  belong,  however,  to  the  monoclinic 
system,  with  only  one  plane  of  symmetry,  as  may  be 
seen  from  the  pictures.  In  Fig.  i  this  eight-sided 
prism  is  terminated  by  a  pair  of  obliquely  placed 
pyramid  planes;  and  in  Fig.  2  there  are  two  such 
pairs  of  pyramid  planes,  together  with  a  small  plane 
at  the  top  of  the  crystals  which  is  perpendicular  to 
the  plane  of  symmetry.  Although  these  Crystals  (of 
augite  and  diopside)  are  monoclinic,  there  are  other 
members  (enstatite,  bronzite,  and  hypersthene)  of 
the  pyroxene  group  which  belong  to  the  orthorhom- 
bic  system;  but  these  only  rarely  occur  as  distinctly 
formed  crystals. 

The  name  pyroxene  has  rather  a  curious  history:  it 
means,  in  Greek,  "a  stranger  to  fire,"  and  was  given 
in  1796  by  the  celebrated  French  mineralogist,  the 
Abbe  Haiiy,  in  the  belief  that  the  crystals  he  so 
named  had  been  accidentally  caught  up  in  the  vol- 
canic lavas  in  which  they  were  found.  As  a  matter 
of  fact,  as  we  now  know,  the  pyroxenes  are  typically 
minerals  of  igneous  origin,  having  crystallized  out 
from  molten  rock-magmas. 


204         THE    WORLD'S    MINERALS 


AUGITE 

(Plate  27,  Fig.  i). — This  variety  of  pyroxene  is 
of  abundant  occurrence  as  an  essential  constituent  of 
basalts,  gabbros,  and  other  dark-colored  (i.e.  basic) 
rocks  of  igneous  origin.  In  the  lavas  of  several  vol- 
canoes, many  of  them  now  extinct,  it  is  found  as  well- 
formed  crystals  embedded  in  the  rock,  as  shown  in 
Fig.  i,  representing  a  fragment  of  lava  from  Bo- 
hemia. When  the  enclosing  rock  is  decomposed  and 
softened  by  weathering,  the  crystals  of  augite  fall 
out,  and  are  often  found  loose  in  the  soil.  These 
crystals  are  of  a  black  or  very  dark  green  color,  with 
dull  surfaces. 

DIOPSIDE 

(Plate  27,  Figs.  2  and  3). — This  is  another  mono- 
clinic  pyroxene,  differing  from  augite  in  being  less 
complex  in  chemical  composition.  Its  formula  may 
be  written  as  CaMg(SiO3)2,  but  very  often  a  small 
amount  of  ferrous  iron  takes  the  place  of  an  equiva- 
lent amount  of  magnesium.  As  the  iron  varies  in 
amount  in  the  isomorphous  mixture,  there  is  a  cor- 
responding variation  in  the  color  of  the  crystals,  from 
almost  white  to  a  dark  green,  the  crystals  shown 
in  Figs.  2  and  3  being  of  an  intermediate  rich  green 
color.  This  variety  of  pyroxene  is  usually  found  as 
well-formed  crystals  in  limestones  which  have  been 
subjected  to  the  baking  action  of  molten  rock-masses. 
The  lustrous  crystals  from  Ala,  in  Piedmont,  rep- 


SILICATES  (Pyroxene  group). 


Plate  27. 


1,  Augite.    2,  3,  Diopside.    4,  Enstatite.    5,  Hypersthene.    6,  Wollastonite. 


BRONZITE— HYPERSTHENE        205 

resented  in  Figs.  2  and  3,  are  found,  together  with 
bright  crystals  of  garnet  (hessonite),  in  veins  travers- 
ing serpentine.  These  clear,  green  crystals  are  some- 
times cut  in  Turin  as  gem-stones. 

BRONZITE 

(Plate  27,  Fig.  4). — The  orthorhombic  pyroxenes 
form  a  series  ranging  in  chemical  composition  from 
magnesium  meta-silicate  (MgSiOs)  to  iron  meta- 
silicate  (FeSiOs) ,  which  mix  together  isomorphously 
»in  all  proportions.  When  but  little  iron  is  present  we 
have  the  mineral  enstatite,  which  is  sometimes  found 
•as  transparent  green  crystals,  suitable  for  cutting  as 
gems.  Bronzite  is  an  intermediate  member  of  this 
series,  with  a  composition  that  may  be  expressed  by 
the  formula  (Mg,Fe)SiO3.  As  represented  in  the 
picture  (Fig.  4),  it  occurs  as  masses  composed  of  a 
confused  aggregate  of  crystalline  individuals,  with  a 
fibrous  structure,  and  exhibiting  a  bronze-yellow 
color  with  a  metallic  sheen  or  luster  (hence  the  name 
bronzite).  These  characters  are  brought  out  to  ad- 
vantage when  the  stone  is  cut  and  polished,  and  it  is 
therefore  sometimes  used  for  ornamental  purposes. 

HYPERSTHENE 

(Plate  27,  Fig.  5) . — This  is  another  member  of  the 
series  of  orthorhombic  pyroxenes,  differing  from 
bronzite  in  containing  less  magnesium  and  more  iron, 
so  that  the  formula  becomes  (Fe,Mg)SiO3.  There 


206        THE    WORLD'S    MINERALS 

being  here  more  iron  in  the  mineral,  its  color  is 
deeper,  being  dark  brown  or  brownish-green.  In 
certain  specimens  the  mineral  exhibits  a  very  pro- 
nounced metallic  reflection,  and  on  this  account  it 
is  cut  and  polished  as  an  ornamental  stone.  This 
character  is  best  shown  by  specimens  from  Labrador, 
where  the  mineral  forms,  together  with  labradorite 
(p.  193),  a  coarse-grained  igneous  rock  called  norite. 

WOLLASTONITE 

(Plate  27,  Fig.  6). — Named  after  the  English 
chemist  and  mineralogist,  W.  H.  Wollaston  (1766- 
1828) ,  this  mineral  of  the  pyroxene  group  was  earlier 
known  as  tabular-spar,  on  account  of  the  plate-like 
shape  of  its  rarely  occurring  crystals.  These  are 
white  in  color,  and  consist  of  calcium  meta-silicate, 
CaSiOa.  The  mineral  is  usually  found  in  metamor- 
phic  limestones,  in  which  it  occasionally  forms  den- 
dritic growths,  as  in  the  black  limestone  near  Pirna, 
in  Saxony  (Fig.  6). 

There  are  a  few  other  minerals  of  the  pyroxene 
group  which,  although  not  represented  on  the  plates, 
are  of  some  importance,  and  also  they  supply  gem- 
stones  of  a  fine  color.  Of  these,  jadeite,  a  silicate  of 
sodium  and  aluminium,  has  already  been  mentioned 
under  the  mineral  nephrite  (p.  199),  which  it  so 
closely  resembles  in  appearance.  Another  is  spodu- 
mene,  a  silicate  of  lithium  and  aluminium,  usually 
found  as  ash-grey  crystals  (hence  the  name),  but 
sometimes  as  transparent  crystals  of  a  beautiful  em- 


THE    SODALITE    GROUP  207 

erald-green  or  violet  color;  the  green  variety  is 
known  as  hiddenite,  and  the  violet,  or  lilac,  as  kunz- 
ite.  A  third  species  is  rhodonite,  a  silicate  of  man- 
ganese (MnSiOs),  so  named  because  of  its  charac- 
teristic rose-red  color.  This  is  found  as  small,  bright 
crystals,  or  as  larger,  suitable  for  cutting  into  slabs. 

THE  SODALITE  GROUP 

This  group  includes  a  few  less  common  minerals, 
which  are  sometimes  of  importance  as  constituents  of 
certain  kinds  of  igneous  rocks.  They  are  complex  in 
chemical  composition,  and  present  the  peculiarity  of 
containing  chloride,  sulphate,  or  sulphide  in  com- 
bination with  the  silicate  portion.  In  crystallization 
they  are  all  cubic;  but  distinctly  formed  crystals  are 
of  rare  occurrence. 

SODALITE 

(Plate  28,  Fig.  i). — This  is  a  silicate  and  chloride 
of  sodium  and  aluminium,  the  name  sodallte  refer- 
ring to  the  presence  of  sodium,  or  soda.  It  is  found 
as  clear,  glassy  crystals  in  the  ejected  bombs  of  Monte 
Somma,  the  ancient  portion  of  Vesuvius.  The  re- 
markably fine  crystals  represented  in  the  picture 
(Fig.  i)  are  distorted  rhombic-dodecahedra,  being 
elongated  in  the  direction  of  one  of  the  triad  axes, 
and  so  presenting  the  appearance  of  hexagonal  prisms 
terminated  by  rhombohedral  planes. 

In  another  form,  of  very  different  appearance,  the 
mineral  occurs  as  compact  masses  of  a  bright  sky-blue 


208        THE    WORLD'S    MINERALS 

color  (very  like  Fig.  2  on  the  same  plate).  These 
masses  are  often  of  considerable  size,  and  form  a 
constituent  of  sodalite-syenite  at  Miask  in  the  Ural 
Mountains,  at  Litchfield  in  Massachusetts,  and  at 
Bancroft  in  Hastings  Co.,  Ontario.  At  the  last- 
named  place  the  mineral  is  quarried,  and  slabs  are 
polished  for  ornamental  purposes.  The  deposits 
were  being  developed  at  the  time  of  the  visit  of  the 
Prince  and  Princess  of  Wales  to  Canada,  and  on 
account  of  the  interest  taken  by  the  Princess  (now 
Queen  Mary)  in  the  stone,  it  came  to  be  known  in 
the  trade  as  Princess  Blue. 

LAPIS-LAZULI 

(Plate  28,  Fig.  2). — Another  mineral  belonging  to 
the  sodalite  group,  and  known  to  mineralogists  as 
lazurlte  (not  to  be  confused  with  lazulite,  p.  178), 
resembles  sodalite  in  composition,  except  that  sulphur 
takes  the  place  of  chlorine.  This  intensely  blue  min- 
eral enters  largely  into  the  composition  of  lapis- 
lazuli,  which  itself  is  really  a  fine-grained  mixture  of 
minerals,  or,  in  other  words,  a  rock,  being  of  the 
nature  of  a  crystalline  limestone  impregnated  with 
lazurite,  sodalite,  iron-pyrites,  etc.  On  this  account, 
and  depending  on  the  amount  of  lazurite  present, 
the  depth  of  color  shown  by  lapis-lazuli  is  somewhat 
variable,  being  usually  a  paler  blue  in  specimens 
from  Lake  Baikal  (Fig.  2)  and  Chile,  and  a  richer 
and  deeper  blue  in  the  more  prized  specimens  from 
Badakshan,  in  central  Asia.  The  stone  is  spotted  and 


SILICATES. 


Plate  28. 


1,  Sodalite.    2,  Lapis-lazuli.    3,  Leucite.    4,  Beryl. 


LAPIS-LAZULI— LEUCITE  209 

veined  with  white  or  yellowish  (iron-stained)  calcite, 
and  speckled  with  grains  of  iron-pyrites.  The  deep- 
blue  ground  set  with  bright,  brassy  specks  of  iron- 
pyrites  suggests  a  comparison  with  the  blue  sky 
bedecked  with  stars. 

As  is  well  known,  lapis-lazuli  is  much  used  for 
small  ornaments,  such  as  beads,  crosses,  etc.,  and  for 
inlaying  in  boxes  and  table-tops.  Being  an  opaque 
stone,  its  beauty  depends  solely  on  its  rich  blue  color. 
When  the  stone  is  crushed  to  powder  and  the- pure 
blue  material  is  separated  by  sedimentation  in  water, 
the  pigment  known  as  ultramarine  is  obtained.  For- 
merly, when  lapis-lazuli  was  the  only  source  of  ultra- 
marine, this  was  very  expensive;  but  now  it  is  manu- 
factured on  a  large  scale  by  heating  in  crucibles  a 
mixture  of  china-clay  (a  hydrated  silicate  of  alu- 
minium) ,  sodium  carbonate,  charcoal,  and  sulphur. 


LEUCITE 

(Plate  28,  Fig.  3). — Well-formed  crystals  of  this 
mineral  are  not  uncommon  in  the  lavas  of  Vesuvius 
and  of  the  extinct  volcanoes  near  Rome  and  in  the 
Eifel.  They  have  a  rounded  appearance,  looking 
like  so  many  angular  peas  embedded  in  the  rock. 
They  are  bounded  by  twenty-four  trapezoidal  faces, 
exactly  like  the  crystals  of  analcite  shown  in  Plate 
36,  Fig.  i.  This  form  is  the  icositetrahedron  of  the 
cubic  system,  the  symbol  being  (112)  (see  p.  14). 


210        THE    WORLD'S    MINERALS 

Crystals  of  leucite  were  long  thought  to  belong  to 
the  cubic  system;  but  when  they  were  examined  in 
thin  sections  under  the  microscope,  it  was  seen  that 
their  internal  structure  and  optical  characters  did  not 
conform  with  cubic  symmetry.  They  really  consist 
of  a  complex  lamellar  intergrowth  of  twinned  ortho- 
rhombic  crystals,  with  the  external  form  of  a  cubic 
crystal.  When,  however,  the  crystals  are  heated  to  a 
temperature  of  714°  C.,  these  peculiarities  of  internal 
structure  disappear,  and  the  crystals  are  then  truly 
cubic;  but  on  their  cooling  again  the  structure  reap- 
pears. We  must,  therefore,  assume  that  when  the 
crystals  were  formed  in  the  red-hot  lava  they  grew  as 
cubic  crystals,  and  that  as  they  cooled  the  cubic  sub- 
stance became  transformed  into  an  orthorhombic  sub- 
stance. We  thus  have  here  a  peculiar  case  of  di- 
morphism, analogous  to  the  dimorphism  of  diamond 
and  graphite  (pp.  49  and  50),  but  one  in  which  the 
cubic  form  can  only  exist  at  a  high  temperature; 
when  cooled  it  changes  spontaneously  into  the  second 
form. 

Chemically,  leucite  is  a  silicate  of  aluminium  and 
potassium,  with  the  formula  KAlSi2Oe,  thus  contain- 
ing the  same  elements  as  potash-felspar,  but  combined 
in  different  proportions.  It  is  found,  sometimes  to- 
gether with  potash-felspar,  in  certain  igneous  rocks 
rich  in  potash.  The  name  leucite  means  "white 
stone" ;  but  this  is  scarcely  an  appropriate  name,  since 
the  crystals  are  usually  of  a  dark  grey  color,  as  shown 
by  the  specimen  from  Vesuvius  (Fig.  3). 


BERYL  211 


BERYL 

(Plate  28,  Fig.  4). — This  mineral  is  a  silicate  of 
the  metals  beryllium  and  aluminium,  and  when  quite 
pure  and  free  from  coloring  matter  and  flaws,  it  is 
colorless  and  clear  like  glass.  Such  crystals  are,  how- 
ever, quite  uncommon;  more  usually  they  are  dull 
and  cloudy  and  of  a  pale  greenish  or  yellowish  color, 
this  color  being  due  to  the  presence  of  traces  of  iron. 
Sometimes  they  are  transparent  and  of  a  rich  grass- 
green  color,  and  we  then  have  the  variety  known  as 
emerald,  which  on  account  of  its  rarity  in  clear  crys- 
tals of  a  good  color  is  one  of  the  most  valuable  of 
gem-stones.  The  color  here  is  probably  due  to  a 
small  amount  of  chromium  oxide.  When  the  color 
of  the  clear  crystals  is  pale  yellowish-green,  bluish- 
green,  or  sea-green,  we  have  the  gem-variety  known 
as  aquamarine.  Other  crystals  are  of  a  yellow 
color,  approaching  golden-yellow.  Recently  very 
fine  beryls  of  a  rich  pink  color  have  been  found  in 
Madagascar  and  California. 

Beryl  crystallizes  in  the  hexagonal  system,  usually 
in  very  simple  form  with  only  the  hexagonal  prism 
and  the  basal  plane  (Fig.  4,  and  Text-Fig.  21,  p.  24) ; 
but  sometimes,  especially  in  the  variety  aquamarine, 
there  is  a  rich  development  of  pyramidal  faces  on  the 
edges  and  corners  between  the  prism  and  the  basal 
plane.  The  habit  of  the  crystals  is  almost  invariably 
prismatic,  and  the  prism  faces  are  striated  in  the 
direction  of  their  length.  Enormous  crystals,  weigh- 


212        THE   WORLD'S    MINERALS 

ing  as  much  as  one  or  two  tons,  have  been  found  at 
Grafton  and  Acworth  in  New  Hampshire,  and  large 
crystals  are  also  found  in  southern  Norway.  The 
specific  gravity  is  2.7,  only  slightly  higher  than  that 
of  quartz ;  so  that  beryl  is  one  of  the  lightest  of  gem- 
stones.  The  hardness  of  7^  is  between  that  of  quartz 
and  topaz. 

Beryl  is  of  common  occurrence  in  some  granites, 
more  particularly  in  the  coarsely  crystallized  veins 
of  pegmatite,  which  traverse  granitic  masses.  Very 
good  transparent  crystals  of  a  blue  color  have  been 
found  in  the  granite  of  the  Mourne  Mountains  in 
County  Down,  and  opaque  crystals  in  the  granite  of 
Counties  Dublin  and  Donegal  and  in  Banffshire. 
The  crystals  embedded  in  white  quartz  in  Fig.  4  are 
from  Bodenmais,  in  Bavaria.  The  emerald  variety 
is  found  in  mica-schist  in  the  Ural  Mountains,  at 
Habachthal  in  Salzburg,  and  at  Jebel  Zabara  in 
Upper  Egypt.  At  the  last-named  locality  the  so- 
called  Cleopatra's  emerald-mines  were  worked  in 
1650  B.  C.f  and  the  stones  cut  as  beads  and  scarabs  by 
the  ancient  Egyptians.  The  best  emeralds,  however, 
all  come  from  Muzo  in  Colombia,  South  America, 
where  they  occur,  with  calcite  and  black  limestone, 
in  crevices  in  clay-slate.  Many  of  the  crystals  are 
much  fissured,  and  stones  perfectly  free  from  flaws 
are  extremely  rare.  The  majority  of  the  emeralds  so 
much  admired  by  Indian  princes  are  doubtless  of 
South  American  origin,  and  many  of  them  perhaps 
formed  part  of  the  spoil  taken  from  the  Peruvians  by 
the  Spanish  conquerors  in  the  sixteenth  century. 


THE    GARNET    GROUP  213 

Aquamarines  of  gem-quality  are  mostly  from  Brazil, 
the  Ural  Mountains,  and  Transbaikalia  in  Siberia. 

THE  GARNET  GROUP 

The  garnets  afford  an  excellent  example  of  an 
isomorphous  group  of  minerals.  They  are  all  alike 
in  their  crystalline  form,  belonging  to  the  cubic  sys- 
tem, and  they  all  have  the  same  type  of  chemical 
formula.  This  formula  may  be  expressed  generally 
as  an  ortho-silicate,  RTsR'"*SuOi»,  where  R"  stands 
for  calcium,  ferrous  iron,  magnesium,  or  manganese; 
and  R'"  stands  for  aluminium,  ferric  iron,  or  chro- 
mium. We  may  thus  have  the  following  kinds  of 
garnet,  which,  it  will  be  seen,  vary  widely  in  their 
actual  chemical  composition : 

Chemical  Mineralogical 
Chemical  Name                                       Formula  Name 

Calcium-iron-garnet Ca3Fe2Si8Oi2  Andradite 

Calcium-chromium-garnet Ca3Cr2Si3Oi2  Uvarovite 

Calcium-aluminium-garnet CasAUSisOu  Grossularite 

Iron-aluminium-garnet FeaAUSisOw  Almandine 

Magnesium-aluminium-garnet   MgsALSisO^  Pyrope 

Manganese-aluminium-garnet MnaALSiaOw  Spessartite 

Although  these  may  be  taken  as  the  types,  it  is  only 
rarely  that  actual  crystals  correspond  exactly  with 
the  compounds  above  stated.  What  actually  hap- 
pens is  that  two  or  more  of  these  compounds  help 
together  to  build  up  one  and  the  same  crystal,  as  has 
already  been  explained  under  isomorphism  (p.  49). 
In  applying  the  mineralogical  names,  we  imply  that 


214        THE   WORLD'S    MINERALS 

the  corresponding  chemical  type  predominates  in  the 
particular  crystal. 

Corresponding  with  these  wide  differences  in 
chemical  composition,  there  must  of  necessity  be  wide 
differences  in  color  (white,  black,  green,  red,  yellow, 
but  not  blue) ,  specific  gravity  (3.4-4.3) ,  and  mode  of 
occurrence.  Amongst  them  we  have  gem-stones  of 
red,  brown,  yellow,  and  green  colors;  while  common 
garnet  is  used  as  an  abrasive  agent  in  the  form  of 
garnet-paper  (sometimes  sold  under  the  name  of 
emery-paper).  The  hardness  is  about  the  same  as 
that  of  quartz,  varying  from  6^2  to  jl/2.  Still  an- 
other use  for  garnets  is  for  gravelling  garden-walks; 
the  small,  rejected  material,  after  picking  out  stones 
large  enough  for  cutting  as  gems,  being  so  used  in  the 
garnet-mining  district  of  Bohemia. 

Crystals  of  garnet  are  always  developed  to  an  equal 
extent  in  all  directions;  that  is,  they  present  neither  a 
prismatic  nor  a  platy  habit,  but  what  may  perhaps  be 
described  as  a  granular  habit.  Most  crystals  found 
embedded  in  rocks  have,  in  fact,  the  form  of  rounded 
grains.  It  is  no  doubt  on  account  of  this  characteristic 
granular  form  that  the  mineral  has  received  its  name. 
The  most  common  crystal  form  is  the  rhombic-dode- 
cahedron, which  for  this  reason  is  sometimes  called 
the  garnetohedron.  The  icositetrahedron,  with  the 
symbol  (112),  as  in  the  crystals  of  leucite  (p.  209) 
and  analcite,  is  also  a  common  form  of  garnet  crystals. 
In  a  combination  of  these  two  forms  the  faces  of  the 
icositetrahedron  truncate  the  edges  of  the  rhombic- 
dodecahedron  (Plate  29,  Figs,  i  and  2). 


HESSONITE— ALMANDINE         215 


HESSONITE 

(Plate  29,  Fig.  i). — Hessonite,  or  cinnamon- 
stone,  is  essentially  a  calcium-aluminium-garnet;  but 
it  always  contains,  in  addition,  small  amounts  of  fer- 
rous and  ferric  iron,  manganese,  and  magnesium;  so 
that  it  is  a  mixture  of  five  of  the  garnet  substances. 
The  crystals  are  often  transparent,  and  of  a  warm 
reddish-brown,  honey-yellow,  or  hyacinth-red  color, 
and  when  cut  with  facets  they  afford  pretty  gems. 
Beautiful  crystals  occur,  in  association  with  clear, 
green  crystals  of  diopside,  in  veins  in  serpentine  at 
Ala,  in  Piedmont.  The  crystals  on  limestone  shown 
in  Fig.  i  are  from  Sweden.  Material  of  the  best 
gem-quality  is  found  as  water-worn  pebbles  in  the 
gem-gravels  of  the  cinnamon  island  of  Ceylon;  the 
name  cinnamon-stone  is,  however,  in  allusion  to  the 
cinnamon-color  of  the  mineral. 

ALMANDINE 

(Plate  29,  Fig.  2). — Almandine  is  essentially  the 
iron-aluminium-garnet,  but,  like  all  the  garnets,  it 
contains  variable  amounts  of  the  other  types  in  its 
mixed  crystals.  Its  color  is  a  deep  rich  red,  often 
with  a  tinge  of  violet.  This  garnet  is  usually  cut  and 
polished  in  rounded  convex  forms  (en  cabochon), 
when  it  often  passes  under  the  name  carbuncle.  Such 
stones  are  cut  in  large  numbers  at  Jaipur,  in  India, 
and  these  Indian-cut  gems  have  much  the  appearance 


216        THE    WORLD'S    MINERALS 

of  jujubes.  Almandine  is  found  at  many  localities, 
occurring  usually  in  mica-schists  and  gneisses.  The 
fine  crystal  embedded  in  mica-schist  shown  in  Fig.  2 
is  from  Fort  Wrangell,  in  Alaska. 

PYROPE 

Pyrope,  or  magnesium-aluminium-garnet,  is  one  of 
the  best-known  varieties,  and  the  one  most  extensively 
cut  as  a  gem-stone.  It  is  of  a  fiery-red  color,  and 
passes  under  a  variety  of  names,  according  to  the  lo- 
cality at  which  it  is  found.  The  best  known  of  these 
is  Bohemian  garnet,  from  the  district  in  northern 
Bohemia,  where  an  important  garnet  mining  and  cut- 
ting industry  has  been  established  for  several  cen- 
turies. Cape  ruby  is  only  another  name  for  pyrope 
from  the  diamond-mines  of  South  Africa.  Elie  ruby 
is  a  pyrope  from  Elie,  in  Fifeshire,  and  Arizona  ruby 
from  Arizona.  Intermediate,  in  chemical  composi- 
tion, between  pyrope  and  almandine  is  the  beautiful 
gem-stone  called  rhodolite,  from  North  Carolina,  the 
color  of  which  is  a  peculiar  and  delicate  rhododen- 
dron-pink. 

ANDRADITE 

Andradite,  the  common  calcium-iron-garnet,  is 
usually  of  a  dark  brown  or  black  color;  but  certain 
specimens  are  yellowish-green  or  emerald-green,  and 
perfectly  transparent.  The  latter  occur  as  rounded 
nodules  in  the  serpentine-rocks  of  the  Urals,  or  as 
pebbles  in  the  neighboring  gold-washings.  They  cut 


SILICATES. 


Plate  29. 


1,  2,  Garnet.    3,  Olivine.    4,  Idocrase. 


GROSSULARITE— UVAROVITE      217 

into  very  effective  and  brilliant  gems,  and  go  under 
the  names  of  demantoid,  or  Uralian  emerald;  but  in 
the  trade  they  are  often  erroneously  called  olivine. 
Being  softer  (H.  =  6^2)  than  the  other  varieties  of 
garnet,  they,  however,  do  not  wear  well  when 
mounted  in  rings. 

GROSSULARITE 

When  chemically  pure,  calcium-aluminium-garnet 
is  colorless;  but  the  small  water-clear  crystals  are 
quite  exceptional.  Usually  the  crystals  are  of  a 
greenish-yellow  color — hence  the  name  grossularite, 
which,  in  Latin,  means  "gooseberry-stone."  Beauti- 
ful rose-pink  crystals  are  found  embedded  in  a  white 
marble  at  Xalostoc,  in  Mexico;  and  with  its  splashes 
of  pale-green  epidote,  this  garnet-bearing  rock  has 
quite  a  pretty  effect  when  cut  into  slabs  and  polished. 

UVAROVITE 

Uvarovite,  or  calcium-chromium-garnet,  is  of  rare 
occurrence  as  small,  brilliant  crystals  of  a  bright 
emerald-green  color.  These  are,  however,  too  small, 
and  wanting  in  transparency,  for  cutting  as  gems. 
This  variety  of  garnet  occurs  in  association  with 
deposits  of  chrome-iron-ore. 


218        THE    WORLD'S    MINERALS 


OLIVINE 

(Plate  29,  Fig.  3). — This  is  a  member  of  another 
isomorphous  group  of  minerals,  in  which  magnesium 
and  iron  replace  each  other  in  the  ortho-silicate  for- 
mula R"2SiC>4.  The  pure  magnesium  ortho-silicate, 
Mg2SiO4,  is  known  as  forsterite,  and  the  pure  iron 
ortho-silicate,  Fe2SiO4,  as  fayalite,  the  intermediate 
members  of  the  series  with  variable  proportions  of 
magnesium  and  iron  being  known  as  olivine.  We  thus 
have  here  a  similar  case  to  that  shown  by  the  mag- 
nesium and  iron  meta-silicates  in  the  group  of  ortho- 
rhombic  pyroxenes.  With  the  entry  of  some  other 
metals,  the  olivine  group  may  also  extend  in  other  di- 
rections, and  several  minerals  of  this  group  are  known. 

In  the  gem-varieties  of  olivine,  and  also  in  most 
rock-forming  olivines,  the  amount  of  magnesium  is 
largely  in  excess  of  the  iron;  and  the  color  of  such 
varieties  is  yellowish-green  or  olive-green.  When 
more  iron,  and  less  magnesium,  is  present  the  color 
may  be  brown.  Crystals  are  of  rare  occurrence,  the 
mineral  being  usually  present  as  irregular  grains  or 
granular  masses  embedded  in  certain  rocks,  of  which 
it  forms  an  important  constituent.  These  rocks  are 
dark  basic  rocks  of  the  basalt,  gabbro,  and  peridotite 
families.  Some  of  the  peridotites  are  composed  al- 
most entirely  of  olivine — for  instance,  the  rock  called 
dunite,  from  the  Dun  Mountain  in  New  Zealand. 
Such  rocks  are  very  liable  to  alteration  when  exposed 
to  weathering  processes;  and  by  the  absorption  of 


OLIVINE— IDOCRASE  219 

water  the  magnesium  silicate  passes  into  the  hydrated 
magnesium  silicate  serpentine.  It  is  in  this  way  that 
the  large  rock-masses  of  serpentine  have  originated. 
An  interesting  occurrence  of  olivine  is  as  a  constitu- 
ent of  meteoric  stones,  and  as  grains  in  some  meteoric 
irons. 

The  bright  crystals  shown  in  Fig.  3,  in  parallel 
grouping,  in  the  cavity  of  a  dark  volcanic  rock  are 
from  Monte  Somma,  Vesuvius ;  but  such  perfect  crys- 
tals are  quite  exceptional.  They  are  bounded  by 
three  pairs  of  pinacoids  at  right  angles  to  one  another, 
three  rhombic  prisms,  and  a  rhombic  pyramid  (the 
small  triangular  faces  on  the  corners). 

Precious  olivine — that  is,  clear,  transparent  ma- 
terial suitable  for  jewelry — is  also  known  by  the 
names  peridot  and  chrysolite.  Its  specific  gravity  is 
3.3;  and  the  hardness  being  only  6%,  the  stone  is 
rather  liable  to  get  scratched  when  worn  in  rings. 
Practically  all  the  material  of  gem-quality  comes 
from  the  small  island  of  St.  John  in  the  Red  Sea, 
where  it  is  found  as  crystals  in  cavities  in  an  altered 
dunite.  The  gem-mining  here  is  a  monopoly  of  the 
Khedive  of  Egypt,  and  is  jealously  guarded.  This 
material  contains  a  small  amount  of  nickel,  to  which 
possibly  the  rich  leaf -green  color  is  partly  due.  Gem- 
material  is  also  found  in  Arizona  and  New  Mexico. 

IDOCRASE 

(Plate  29,  Fig.  4). — Although  this  mineral  is  very 
simple  in  its  crystalline  form,  it  is  extremely  complex 


220        THE    WORLD'S    MINERALS 

in  its  chemical  composition.  It  is  essentially  a  sili- 
cate of  calcium  and  aluminium,  but  many  other  ele- 
ments are  also  present.  Crystals  are  common,  and  are 
usually  well  developed.  Those  represented  in  Fig. 
4  consist  of  a  combination  of  two  square  (tetragonal) 
prisms,  a  tetragonal  pyramid,  and  the  small,  square 
basal  plane  at  the  top.  The  brown  crystals  here  de- 
picted are  from  Monte  Somma,  Vesuvius,  where 
they  occur  plentifully  in  the  ejected  blocks  of  the  old 
volcano.  From  this  well-known  occurrence  the  min- 
eral is  often  known  as  vesuvianite.  Crystals  of  a 
bright-green  color  are  found  at  Ala,  in  Piedmont,  and 
these  are  occasionally  cut  as  gems  at  Turin.  At  other 
localities  the  mineral  is  usually  found  in  metamor- 
phic  limestones.  Large  blocks  of  massive  idocrase  of 
a  rich  green  color  and  with  a  marked  degree  of  trans- 
lucency  have  recently  been  found  in  California,  this 
material  being  cut  and  polished  for  a  variety  of  small 
ornaments  under  the  name  of  californite. 

TOPAZ 

(Plate  30,  Figs,  i  and  2). — This  well-known  gem- 
stone  presents  a  wide  range  of  colors.  Frequently  it 
is  perfectly  colorless  and  water-clear,  and  pebbles  of 
such  material  are  known  to  the  Brazilians  as  pingos 
d'agua  (drops  of  water).  The  popular  idea  of  topaz 
is,  however,  a  stone  of  a  sherry-yellow  color,  this 
being  the  common  Brazilian  topaz  formerly  much 
used  in  jewelry;  but  when  this  stone  is  heated  it 
changes  in  color  to  a  rose-pink,  being  then  known  as 


SILICATES. 


Plate  30. 


1,  2,  Topaz. 


TOPAZ  221 


burnt  topaz.  Other  stones,  which  also  may  be  from 
Brazil,  are  of  delicate  blue,  bluish-green,  or  smoke- 
brown  colors. 

Topaz  is  a  fluo-silicate  of  aluminium  with  the 
chemical  formula  (AlF)2SiO4;  but  the  fluorine  is 
often  partly  replaced  by  the  elements  of  water  in  the 
form  of  hydroxyl  (OH).  The  mineral  is  usually 
met  with  as  well-formed  crystals,  but  sometimes  it  is 
found  as  water-worn  crystals  and  pebbles.  These  are 
orthorhombic,  with  a  prismatic  habit,  and  the  prism 
faces  are  striated  in  the  direction  of  their  length. 
The  prism  invariably  consists  of  a  combination  of 
two  vertical  rhombic  prisms,  terminated  by  horizon- 
tal prisms  or  domes,  pyramids,  and  frequently  also  by 
the  basal  plane.  The  two  crystals  in  Fig.  2  are  each 
bounded  by  the  two  vertical  prisms,  two  horizontal 
prisms,  two  pyramids,  and  the  base.  In  Fig.  i  the 
regularity  of  the  crystal  is  somewhat  interrupted  by 
parallel  grouping;  but  the  forms  here  are  the  same  as 
before,  except  that  only  one  horizontal  prism  (the 
large  triangular  face  to  the  front)  is  present;  on  the 
left  are  four  faces  of  two  pyramids,  but  on  the  right 
the  two  faces  of  only  one  pyramid. 

A  very  important  character  of  crystals  of  topaz  is 
the  perfect  cleavage  in  one  direction  parallel  to  the 
basal  plane.  The  crystals  are  usually  grown  attached 
at  one  end  to  the  matrix,  as  in  Fig.  2,  and  when  they 
are  detached  from  this  they  break  away  with  a 
smooth  plane  surface  along  the  cleavage;  this  is 
shown  by  the  even  underside  of  the  detached  crystal 
in  Fig.  i. 


222        THE    WORLD'S    MINERALS 

The  specific  gravity  of  topaz  is  about  the  same  as 
that  of  diamond — namely,  3.5;  but  the  hardness 
(H.  =  8)  and  brilliancy  of  luster  are  much  lower. 
The  crystals  are  often  perfectly  transparent,  with 
bright  faces;  but  large  crystals  are  frequently  opaque 
and  dull.  For  instance,  a  Norwegian  crystal,  two  feet 
in  length  and  weighing  137  lb.,  shown  in  the  mineral 
gallery  of  the  British  Museum,  is  quite  opaque  and 
rough.  Another  noteworthy  specimen  to  be  seen  in 
the  same  collection  of  minerals  is  a  water-worn  peb- 
ble of  clear,  colorless  topaz,  weighing  very  nearly 
13  lb.  Years  ago  this  block  had  been  used  as  a  door- 
step at  a  shop  in  Fleet  Street  in  London.  By  the  man 
in  the  street  it  would  no  doubt  be  regarded  as  a  lump 
of  glass  and  not  worth  carrying  away;  but  the  critical 
eye  of  a  mineralogist  noticed,  that  on  two  opposite 
sides  of  the  block  there  are  plane  and  smooth  surfaces 
of  fracture,  these  being,  of  course,  due  to  the  perfect 
basal  cleavage  so  characteristic  of  topaz. 

Topaz  occurs  in  nature  in  veins  of  tin-ore  (cas- 
siterite),  and  under  these  conditions  small  colorless 
crystals  are  found  in  some  of  the  Cornish  tin-mines, 
and  in  the  stream-tin  deposits  of  New  South  Wales 
and  Japan.  The  larger  crystals  occur  in  crystal-lined 
cavities  in  granites  and  pegmatites.  That  shown  in 
Fig.  i  is  from  the  granitic  rocks  at  Alabashka,  near 
Ekaterinburg,  in  the  Ural  Mountains.  Other  well- 
known  Russian  occurrences  are  in  the  Ilmen  Moun- 
tains and  the  Sanarka  River  (here  as  red  crystals  in 
the  gold-washings)  in  the  southern  Urals,  and  in 
Transbaikalia  in  Siberia.  One  of  the  localities  in 


TOPAZ— ANDALUSITE  223 

Transbaikalia  rejoices  in  the  name  of  the  Borsch- 
chovochnoi  Mountains ;  some  very  fine  crystals  from 
there  are  preserved  in  the  British  Museum  collection ; 
but,  as  they  lose  their  delicate  brown  color  on  ex- 
posure to  light,  they  are  kept  under  cover.  The 
crystals  shown  in  Fig.  2  are  from  the  Thomas  Range 
in  Utah,  where  they  occur  in  cavities  in  a  volcanic 
rock  called  rhyolite.  Much  of  the  gem-material  of 
various  colors  comes  from  Minas  Geraes,  in  Brazil. 

ANDALUSITE 

(Plate  31,  Figs.  1-3). — This  is  a  silicate  of  alu- 
minium, Al2SiO5,  consisting  of  a  combination  of  one 
molecule  of  alumina  (AUOs)  with  one  of  silica 
(SiO2).  It  is,  however,  not  the  only  mineral  with 
this  chemical  composition;  there  are  two  others — 
namely,  kyanite  (Fig.  4)  and  sillimanite.  These 
three  mineral  species  are  thus  trimorphous  forms  of 
the  same  chemical  compound,  differing  from  one  an- 
other in  crystalline  form  and  physical  characters. 

Andalusite  forms  simple  rhombic  prisms,  with  an 
angle  not  far  from  90°,  which  are  terminated  at  right 
angles  to  their  length  by  the  basal  pinacoid  (Fig.  3) ; 
they  belong  to  the  orthorhombic  system.  Usually  the 
crystals  are  opaque  and  of  a  dull  grey  color;  and 
they  are  often  coated  on  their  surface  with  a  film 
of  mica  (Fig.  3) ,  which  has  resulted  from  their  alter- 
ation. The  crystals  shown  in  Fig.  3  are  embedded  in 
a  vein  of  white  quartz  traversing  mica-schist,  and  are 
from  the  Lisens-Alp,  in  the  Tyrol.  Specimens  were 


224        THE    WORLD'S    MINERALS 

first  collected  at  the  end  of  the  eighteenth  century 
in  Andalusia,  and  it  is  from  this  locality  that  the 
mineral  takes  its  name.  The  same  mineral  is  also 
found  as  transparent  pebbles  in  the  gem-gravels  of 
Minas  Geraes,  Brazil.  These  are  remarkable  for 
their  very  strong  pleochroism:  when  looked  through 
in  different  directions  they  exhibit  different  colors — 
olive-green  and  blood-red.  When  faceted  as  gems, 
such  stones  present  a  very  striking  effect 

A  peculiar  variety  of  andalusite  is  that  known  as 
chiastolite,  or  cross-stone.  This  contains  regularly 
arranged  enclosures  of  black  carbonaceous  matter, 
which  has  been  caught  up  by  the  crystals  during  their 
growth.  When  these  crystals  are  cut  in  slices  per- 
pendicularly to  the  length  of  the  prism,  the  cross- 
sections  show  a  pattern,  either  a  black  cross  on  a 
dirty  white  or  yellowish  ground  (Fig.  i)  or  a  white 
cross  on  a  black  ground  (Fig.  2),  depending  on  the 
relative  amounts  of  the  black  enclosures.  The  crys- 
tals represented  in  the  two  pictures  have  been  so  cut 
and  polished.  Polished  slices  are  often  mounted  and 
worn  as  charms.  These  crystals  occur  embedded  in 
clay-slates  near  the  contact  of  these  rocks  with  a  mass 
of  granite,  and  they  have  been  formed  by  the  baking 
action  of  the  molten  igneous  mass  when  it  was  in- 
truded into  the  slates.  The  specimens  represented  in 
Figs,  i  and  2  are  from  Lancaster,  in  Massachusetts, 
but  similar  specimens  are  found  at  several  other 
places.  Small  crystals  can  be  collected  in  abundance 
from  the  slates  surrounding  the  granite  of  Skiddaw, 
in  Cumberland. 


SILICATES. 


Plate  31. 


1-3,  Andalusite  (1—2,  var.  Chiastolite).    4,  Kyanite. 


KYANITE  225 


KYANITE 

(Plate  31,  Fig.  4). — Though  identical  with  an- 
dalusite  in  chemical  composition,  this  mineral  is 
quite  different  in  appearance.  It  occurs  as  bladed 
crystals  of  a  sky-blue  color;  and  it  is  named  kyanite, 
or  cyanite,  on  account  of  this  characteristic  color. 
The  crystals  belong  to  the  anorthic  system,  but  they 
only  rarely  show  any  other  form  than  the  very  charac- 
teristic blades.  They  have  a  perfect  cleavage  parallel 
to  the  surface  of  the  blades,  and  running  transversely 
across  this  cleavage  surface  are  fine  lines  due  to  the 
presence  of  secondary  twinning  (shown  at  the  top  in 
Fig.  4),  just  as  in  crystals  of  stibnite  (p.  79).  This 
mineral  is  of  special  interest  to  mineralogists  by  rea- 
son of  its  peculiarities  of  hardness.  On  the  bladed 
cleavage  surface  in  a  direction  perpendicular  to  the 
length  the  hardness  is  7,  but  in  the  direction  of  the 
length  of  the  crystal  it  is  only  5.  This  surface  can 
thus  be  scratched  by  quartz  in  one  direction,  but  not 
in  the  other. 

The  bladed  crystal  in  a  matrix  of  white  mica-schist 
represented  in  Fig.  4  is  from  Monte  Campione,  in 
the  St.  Gotthard  district,  in  Switzerland.  Good 
specimens  are  also  found  at  Botriphnie,  in  Banffshire, 
and  at  Carrowstrasna,  in  County  Donegal.  Clearer 
material  of  a  deep  blue  color  is  occasionally  cut  as  a 
gem-stone. 


226        THE    WORLD'S    MINERALS 


EPIDOTE 

(Plate  32,  Fig.  i). — This  is  a  complex  silicate  of 
aluminium,  iron,  and  calcium,  with  a  small  propor- 
tion of  water;  but  owing  to  isomorphous  mixing  the 
exact  composition  is  somewhat  variable.  Crystals  are 
monoclinic,  with  the  peculiarity  that  they  are  usually 
elongated  in  the  direction  perpendicular  to  the  single 
plane  of  symmetry.  Only  rarely  can  the  forms  of 
the  crystals  be  made  out  on  inspection;  they  are 
usually  rod-like  or  needle-like,  and  arranged  in  con- 
fused or  divergent  bundles  (Fig.  i).  They  possess 
a  perfect  cleavage  in  one  plane  direction  parallel  to 
their  length.  One  of  the  most  characteristic  features 
of  common  epidote  is  its  peculiar  shade  of  yellowish- 
green  color,  which  is  compared  to  that  of  the  pis- 
tachio-nut, and  on  this  account  the  mineral  is  some- 
times known  as  pistacite.  Sometimes,  however,  the 
mineral  is  brown  in  color.  Epidote  occurs  in  schists 
and  other  metamorphic  rocks,  and  has  been  formed 
by  the  alteration  of  various  minerals,  such  as  felspar, 
hornblende,  etc.  A  rock  composed  entirely  of  epi- 
dote and  quartz  is  called  an  epidote-schist,  and  it  is 
in  the  crevices  of  such  rocks  that  the  finest  crystals  of 
epidote  are  found,  such  as  those  from  the  Knappen- 
wand,  in  Untersulzbachthal,  Salzburg  (Fig.  i).  The 
green  crystals  from  this  locality  are  occasionally  cut 
as  gem-stones. 


SILICATES. 


Plate  32. 


1,  Epidote.    2,  Axinite.    3,  Prehriite1. : 


AXINITE— PREHNITE  227 


AXINITE 

(Plate  32,  Fig.  2). — Axinite  takes  its  name  from 
the  characteristic  axe-like  shape  of  its  anorthic  crys- 
tals, all  the  faces  of  which  are  obliquely  inclined  to 
one  another.  There  being  only  a  center  of  symmetry, 
each  simple  form  on  the  crystals  consists  only  of  a 
pair  of  parallel  faces;  but  it  will  be  noticed  from  the 
picture  that  these  are  arranged  in  zones  around  the 
crystal,  with  series  of  parallel  edges.  The  charac- 
teristic color  is  clove-brown,  and  the  crystals  are 
sometimes  glassy  and  transparent,  being  then  oc- 
casionally cut  as  gem-stones.  Chemically,  the  mineral 
is  a  silicate  of  aluminium,  calcium,  manganese,  and 
iron,  with  some  boron  and  a  little  water.  It  is  of 
metamorphic  origin,  and  is  usually  found  in  crevices 
in  hornblende-schist  or  in  diabase.  The  best  crystals 
are  from  Bourg  d'Oisans,  in  Dep.  Isere,  France  (Fig. 
2) ;  but  good  specimens  are  also  found  at  several 
places  in  Cornwall. 

PREHNITE 

(Plate  32,  Fig.  3). — This  is  a  silicate  of  alumin- 
ium and  calcium,  containing  a  small  proportion  of 
water.  It  crystallizes  in  the  orthorhombic  system, 
but  distinctly  formed  crystals  are  extremely  rare. 
The  individual  crystals  are  closely  aggregated  in 
radiating  forms,  with  rounded  external  surfaces;  the 
botryoidal  forms  (  like  a  bunch  of  grapes)  shown  in 
Fig.  3  being  especially  characteristic  of  the  mineral 


228        THE    WORLD'S    MINERALS 

when  taken  in  conjunction  with  the  pale  green  color. 
Prehnite  is  a  mineral  of  secondary  origin,  occurring 
in  cavities  of  basic  igneous  rocks,  such  as  basalt  and 
diabase,  and  often  in  association  with  the  zeolites, 
with  which,  indeed,  it  is  sometimes  classified.  Fine 
specimens  are  found  in  the  ancient  volcanic  rocks  at 
several  places  in  the  neighborhood  of  Edinburgh  and 
Glasgow,  and  in  those  of  Paterson,  in  New  Jersey 
(Fig.  3).  The  mineral  is  also  found  in  the  Lake 
Superior  copper-mines,  and,  together  with  axinite,  at 
Bourg  d'Oisans,  in  the  French  Alps.  Large  masses 
were  long  ago  met  with  at  Cradock,  in  Cape  Colony, 
the  mineral  having  been  first  found  there  towards  the 
end  of  the  eighteenth  century  by  Colonel  Prehn,  the 
governor  of  the  Dutch  colony  of  the  Cape  of  Good 
Hope,  and  it  was  consequently  named  in  honor  of 
him. 

TOURMALINE 

(Plate  33,  Figs.  1-3). — Owing  to  isomorphous  re- 
placements, this  mineral  varies  so  widely  in  chemical 
composition  that  we  are  really  dealing  with  a  group 
of  isomorphous  minerals  rather  than  a  single  min- 
eral. In  addition  to  silicon,  aluminium,  and  boron, 
it  contains  smaller  and  variable  amounts  of  iron,  mag- 
nesium, lithium,  sodium,  water,  fluorine,  etc.  It  is, 
therefore,  theoretically  possible  to  distinguish  as 
chemical  varieties  iron-tourmaline,  magnesium-tour- 
maline, and  lithium-tourmaline;  but  such  a  classifi- 
cation is  of  little  practical  value,  since  these  differ- 
ences in  chemical  composition  are  not  associated  with 


TOURMALINE  229 

any  marked  difference  in  physical  character  which 
would  enable  us  to  distinguish  readily  between  one 
variety  and  another.  To  make  a  chemical  analysis  of 
tourmaline  involves  a  long  and  tedious  series  of  op- 
erations; and  if  only  a  small  gem-stone  were  available 
for  examination  the  whole  of  the  material  might  be 
used  up  in  arriving  at  a  decision  as  to  its  nature.  It 
has,  however,  long  been  the  custom  to  distinguish 
differently  colored  gem-varieties  of  tourmaline  by 
special  names:  the  red  as  rubellite,  blue  as  indicolite, 
green  as  Brazilian  emerald,  brown  as  dravite,  color- 
less as  achroite,  while  black  tourmaline  is  commonly 
known  as  schorl. 

Well-shaped  crystals,  belonging  to  the  rhombo- 
hedral  system,  are  not  uncommon,  but  often  the  min- 
eral tends  to  form  radiating  aggregates  of  fine 
needles.  An  interesting  feature  of  the  crystals  is  their 
hemimorphic  development,  the  two  ends  being  pro- 
vided with  different  crystal-forms.  This  is  shown  by 
the  long  prismatic  crystal  in  Fig.  i,  there  being  an 
obtuse  triangular  pyramid  at  the  top  and  a  more 
acute  triangular  pyramid  at  the  lower  end.  The 
shape  of  the  cross-section  of  the  prism  is  also  a  very 
characteristic  feature  of  the  mineral.  The  prism  is 
sometimes  simply  hexagonal,  as  shown  in  Fig.  2,  but 
more  often  this  hexagonal  prism  is  combined  with  a 
trigonal  (three-faced)  prism;  in  Fig.  i  the  long, 
narrow  faces  are  those  of  the  hexagonal  prism,  and 
the  wide  face  seen  to  the  front  is  one  of  the  three 
faces  of  the  trigonal  prism.  Now  in  most  crystals  of 
tourmaline  the  prism  faces  are  so  deeply  striated  and 


230        THE   WORLD'S    MINERALS 

grooved  parallel  to  their  length  (an  indication  of 
this  is  seen  in  Fig.  2)  that  the  nine  (6  +  3)  faces 
round  into  one  another,  and  we  then  have  a  cross- 
section  of  the  shape  of  a  triangle,  with  curving  con- 
vex sides.  This  character  alone  is  usually  sufficient 
to  enable  us  to  identify  a  mineral  as  tourmaline. 

This  hemimorphic  or  two-ended  character  of  crys- 
tals of  tourmaline  is  an  outward  expression  of  a  pe- 
culiarity in  the  internal  structure  of  the  crystalline 
material,  there  being  a  polarity  in  the  arrangment  of 
the  crystalline  particles  building  up  the  crystal.  As 
a  consequence  of  this,  crystals  of  tourmaline  behave 
in  a  peculiar  manner  when  they  are  subjected  to 
changes  of  temperature.  If  a  crystal  be  warmed  it 
develops  a  charge  of  positive  electricity  at  one  of  its 
ends,  and  a  charge  of  negative  electricity  at  the  other; 
while  if  it  be  cooled  these  charges  are  reversed.  The 
presence  of  these  electrical  charges  can  be  demon- 
strated in  a  very  pretty  and  conclusive  manner  by  the 
following  simple  experiment.  A  mixture  of  red-lead 
and  flowers  of  sulphur  is  dusted  through  a  fine  sieve 
over  a  cooling  crystal  of  tourmaline,  when  it  will  be 
seen  that  one  end  of  the  crystal  becomes  red  and  the 
other  yellow.  The  reason  of  this  is  that  the  sulphur 
is  attracted  to  the  positively  charged  end,  and  the 
red-lead  to  the  negatively  charged  end  of  the  crystal. 

This  pyro-electric  property  of  tourmaline  was  first 
noticed  by  the  Dutch  in  1703.  Some  crystals  of  the 
mineral  which  had  been  brought  from  Ceylon  had 
fallen  amongst  the  hot  ashes  of  a  peat  fire,  and  it  was 
found  that  the  ashes  adhered  to  the  crystals.  The 


TOURMALINE  231 

mineral  was  consequently  called  aschtrekker,  mean- 
ing, in  Dutch,  "ash-drawer."  It  is  interesting  to  note 
also  that  the  name  tourmaline  dates  from  the  same 
period,  being  a  corruption  of  the  Cingalese  word 
turamali. 

Another  interesting  property  of  tourmaline  is  one 
depending  on  its  very  strong  dichroism.  One  of  the 
two  rays — into  which  a  single  ray  of  light  is  split  on 
entering  the  crystal — is  almost  entirely  absorbed,  es- 
pecially in  the  darker  colored  crystals;  a  plate  of 
tourmaline,  cut  parallel  to  the  prism  edges,  may 
therefore  be  used  for  producing  plane-polarized 
light.  Two  such  plates  when  placed  in  crossed  po- 
sition over  one  another  almost  completely  cut  out  the 
light;  while  one  by  itself  or  the  two  placed  together 
in  parallel  position  are  transparent.  On  this  depends 
the  use  of  the  little  piece  of  apparatus  known  as  tour- 
maline tongs.  In  some  crystals  the  absorption  is  so 
great  that  a  section  cut  parallel  to  the  basal  plane  will 
be  quite  opaque,  while  a  much  thicker  section  cut 
parallel  to  the  prism  edges  may  transmit  light.  The 
lapidary  must  take  into  account  this  absorptive  action 
of  tourmaline,  and  he  must  cut  a  faceted  stone  in  such 
a  direction  from  the  crystal  that  the  large  table  facet 
at  the  front  shall  be  parallel  to  the  direction  of  the 
prism  edges. 

As  already  mentioned,  the  color  of  tourmaline 
[Varies  widely;  but  we  may  also  have  a  very  wide 
range  of  color  in  one  and  the  same  crystal,  there  often 
being  zones  of  color,  either  perpendicular  (Fig.  2) 
or  parallel  to  the  length  of  the  prism.  Some  crystals 


232        THE    WORLD'S    MINERALS 

from  the  island  of  Elba,  of  a  pale  pink  color,  are 
tipped  with  jet-black,  and  are  consequently  described 
as  "negro-heads."  The  luster  of  tourmaline  is  glassy, 
and  the  crystals  possess  no  cleavage.  The  specific 
gravity  varies  from  3.0  to  3.2,  and  the  hardness 
(H.  =  7^2 )  is  rather  greater  than  that  of  quartz. 

Tourmaline  occurs  in  granites,  mica-schists, 
gneisses,  and  crystalline  limestones;  and  mixed  with 
quartz  it  sometimes  forms  large  rock-masses.  It  is  a 
constant  associate  of  cassiterite  in  veins  of  tin-ore, 
black  tourmaline  being  a  very  common  mineral  in 
the  Cornish  tin-mines.  Most  of  the  gem-varieties 
are  found  as  crystals  in  cavities  in  pegmatite-veins 
traversing  granite,  and  they  are  obtained  by  quarry- 
ing these  rocks  in  California,  Madagascar,  and  the 
Ural  Mountains.  The  black  crystals  embedded  in 
white  quartz  shown  in  Fig.  i  are  from  Horlberg,  near 
Bodenmais,  in  Bavaria;  the  crystal  in  Fig.  2  is  from 
Haddam,  in  Connecticut;  and  the  radiating  groups 
of  pink  tourmaline  (rubellite),  embedded  in  pale 
lilac-colored  lepidolite  (Fig.  3),  from  Pala,  in  San 
Diego  Co.,  California. 

STAUROLITE 

(Plate  33,  Figs.  4  and  5). — Like  the  name  chiasto- 
lite  (p.  224),  the  name  staurolite  also  means  "cross- 
stone."  Here,  however,  instead  of  showing  a  colored 
cross  inside  the  crystals,  the  cross-shaped  form  is  pro- 
duced by  the  regular  intergrowth  of  two  crystals  in 
twinned  position  (Fig.  5).  The  crystals  are  ortho- 


SILICATES. 


Plate  33. 


1—3,  Tourmaline.    4,  5,  Staurolite. 


7,  I  J  f  r. 


THE    MICA    GROUP  233 

rhombic,  and  are  bounded  by  a  rhombic  prism  and 
the  basal  pinacoid.  They  are  of  a  dark  brown  color, 
and  usually  dull  and  opaque.  Chemically,  the  mineral 
is  an  extremely  complex  silicate  of  aluminium,  iron, 
etc.  It  is  found  as  embedded  crystals  in  mica-schist, 
as  shown  in  the  specimen  (Fig.  4)  from  Brittany. 
The  isolated  twinned  crystal  (Fig.  5)  is  from  Fannin 
Co.,  Georgia. 

THE  MICA  GROUP 

The  micas  are  not  only  of  considerable  importance 
as  rock-forming  minerals,  but  they  also  find  many 
useful  applications.  As  essential  constituents  of 
igneous  and  crystalline  rocks  of  many  kinds  they 
have  an  extremely  wide  distribution.  Granite,  for 
instance,  is  a  rock  composed  of  quartz,  felspar,  and 
mica;  and  mica-schist  is  a  foliated  rock,  consisting 
of  quartz  and  mica.  When  these  rocks  are  broken 
down  by  the  action  of  weathering  agents,  and  their 
materials  transported  by  running  water  to  be  de- 
posited in  lakes  or  seas  and  so  form  new  rocks,  the 
mica,  though  broken  up  mechanically  into  fine  scales, 
resists  any  chemical  actions,  and  it  consequently  en- 
ters into  the  composition  of  the  newly  formed  bedded 
rocks.  If  we  examine  a  sandstone  with  a  magnifying 
lens  we  shall  see  on  the  surface  of  the  bedding  planes, 
along  which  the  stone  splits,  numerous  shining  sil- 
very scales  of  mica.  In  some  sandstones  these  scales 
of  mica  are  very  conspicuous;  while  in  finer  rocks, 
such  as  clays,  they  are  very  minute. 


234        THE    WORLD'S    MINERALS 

The  micas  almost  always  crystallize  as  platy  or 
scale-like  crystals,  and  when  these  have  been  free  to 
develop  at  their  edges  they  always  have  a  six-sided 
outline  (Plate  34,  Figs,  i  and  3),  with  angles  differ- 
ing only  very  slightly  from  1 20°.  To  all  appearances, 
therefore,  the  crystals  are  hexagonal;  but  exact  meas- 
urements of  the  angles  between  the  faces  of  perfectly 
developed  crystals,  and  an  examination  of  their  op- 
tical characters,  prove  that,  in  reality,  they  belong  to 
the  monoclinic  system. 

A  very  important  feature  of  these  crystals  is  the 
possession  of  a  perfect  cleavage  in  one  plane  direction 
parallel  to  their  large,  flat  surface.  In  this  direction 
the  crystals  can  be  split  up  indefinitely  into  the  thin- 
nest of  leaves,  with  perfectly  smooth  and  bright 
surfaces.  At  the  mica-mines  the  large  crystals  are 
called  books,  and  these  are  split  into  leaves  in  pre- 
paring the  material  for  the  market.  Owing  to  the 
presence  of  this  highly  perfect  cleavage,  the  flat  sur- 
faces of  mica  display  a  pronounced  pearly  or  silvery 
luster,  and  also  very  frequently  colored  bands  of  the 
same  nature  as  Newton's  rings.  Being  such  a  very 
characteristic  feature  of  mica,  a  cleavage  of  this  high 
degree  of  perfection  is  spoken  of  as  a  micaceous 
cleavage,  and  as  such  is  very  often  applied  as 
a  descriptive  term  in  connection  with  certain  other 
minerals. 

The  thin  cleavage  flakes  split  from  a  crystal  of 
mica  can  be  readily  bent,  and  when  released  they 
spring  back  to  their  original  position;  that  is,  they 
are  both  flexible  and  elastic.  The  material  is  not 


THE    MICA    GROUP  235 

hard  (H.  =  2>£,  about),  and  the  smooth  cleavage 
surfaces  can  be  scratched  with  the  finger-nail,  though 
not  so  readily  as  can  talc  and  gypsum.  When  a  sheet 
of  mica  is  struck  a  smart  blow  with  a  sharp-pointed 
instrument,  a  six-rayed  star  of  fracture,  called  a  per- 
cussion figure,  is  produced. 

In  their  chemical  composition  the  micas  present  a 
certain  analogy  to  tourmaline;  and  it  is  an  interesting 
fact  that,  under  certain  conditions  in  the  earth's  crust, 
tourmaline  becomes  altered  to  mica.  The  same  chem- 
ical elements  are  present,  with  the  exception  of  boron. 
The  micas  are  essentially  silicates  of  aluminium,  to- 
gether with  water  and  alkali  metals,  and  sometimes 
magnesium,  iron,  and  fluorine.  As  with  tourmaline, 
so  here  we  may  distinguish  several  chemical  varieties 
— namely,  potassium-mica,  sodium-mica,  lithium- 
mica,  magnesium-mica,  and  iron-mica.  These  differ- 
ences in  chemical  composition  (amongst  the  micas, 
much  more  so  than  in  tourmaline)  are  accompanied 
by  marked  differences  in  physical  characters,  espe- 
cially with  respect  to  the  optical  properties;  and  it  is 
thus  possible,  and  also  advantageous,  to  distinguish 
the  different  chemical  varieties  by  special  names. 
The  most  important  of  these  are  mentioned  below; 
others  are  paragonite,  or  sodium-mica,  which  occurs 
as  very  fine,  snow-white  scales  in  certain  mica-schists 
(for  example,  as  the  matrix  of  kyanite  in  Plate  31, 
Fig.  4)  ;  and  lepidomelane,  or  iron-mica,  which  is 
jet-black  in  color. 


236        THE    WORLD'S    MINERALS 


MUSCOVITE 

(Plate  34,  Fig.  i). — This  is  the  potassium-mica; 
it  is  the  most  abundant,  and  at  the  same  time  the 
chemically  simplest,  member  of  the  group.  Its  com- 
position is  expressed  by  the  formula  HUKA^SiO^a. 
The  hydrogen  here  plays  the  part  of  a  metal  in  the 
silicate  formula;  it  is  expelled  as  water  only  when 
the  mineral  is  heated  to  a  very  high  temperature. 

The  larger  crystals  are  usually  of  a  dark  greyish 
shade  of  color,  but  cleavage  flakes  are  often  quite 
colorless  and  transparent.  This  transparency  of  the 
cleavage  sheets  has  a  connection  with  the  name  mus- 
covite,  which  is  merely  the  mineralogical  form  of  the 
old  and  popular  name  Muscovy  glass,  clear  sheets  of 
this  mica  having  formerly  been  used  instead  of  glass 
for  windows  in  Russia.  A  variety  known  as  fuchsite 
is  bright  green,  this  color  being  due  to  the  presence 
of  chromium. 

As  small  scales,  muscovite  is  of  abundant  occur- 
rence in  all  countries;  but  it  is  only  in  the  pegmatite- 
veins  of  certain  localities  that  large  crystals  and  sheets 
are  found.  These  are  mined  in  India,  the  United 
States,  and  Brazil.  The  crystal  embedded  in  white 
quartz  shown  in  Fig.  i  is  from  the  felspar-mines  near 
Kragero,  in  southern  Norway. 

Of  the  many  useful  applications  to  which  mus- 
covite— as  well  as  some  other  kinds  of  mica — is  put, 
the  following  may  be  indicated:  for  the  windows  of 
stoves  and  lanterns,  the  peep-holes  of  furnaces,  and 


PHLOGOPITE— BIOTITE  237 

the  chimneys  of  incandescent  gas-burners.  When 
used  for  these  purposes  the  material  is  often  popu- 
larly called  talc;  but  talc  is  quite  a  distinct  mineral, 
to  be  described  presently.  Powdered  mica  is  used 
for  producing  a  spangled  effect  on  toys,  stage  scenery, 
wall-paper,  etc.,  and  as  a  lubricant.  At  the  present 
day,  sheets  of  mica  are  extensively  used  in  the  con- 
struction of  electrical  apparatus  and  machinery. 

PHLOGOPITE 

Phlogopite,  or  magnesium-mica,  contains  the  ele- 
ments magnesium  and  fluorine  in  addition  to  those 
present  in  muscovite.  It  much  resembles  muscovite 
in  appearance,  but  is  often  of  a  yellowish  or  brownish 
shade  of  color,  and  has  a  more  pronounced  silvery 
appearance.  It  differs  also  in  its  mode  of  occurrence, 
being  usually  found  in  crystalline  limestones.  The 
mica  extensively  mined  in  Canada  and  Ceylon  is  of 
this  variety.  It  is  used  for  the  same  purposes  as  is 
muscovite. 

BIOTITE 

(Plate  34,  Fig.  2). — This  differs  from  phlogopite 
in  containing  some  iron  in  addition  to  magnesium;  it 
is  consequently  darker  in  color,  being  deep  brown  or 
black.  Owing  to  this  color  it  is  of  less  commercial 
value.  It  is  a  common  constituent  of  some  granites 
and  gneisses.  In  a  kind  of  granite  known  as  two  mica 
granite,  the  scales  of  light-colored  mica  are  musco- 
vite, and  the  dark-colored  scales  are  biotite.  Less 


238        THE    WORLD'S    MINERALS 

frequently  it  is  found  as  large  sheets;  the  cleavage 
flake  of  irregular  outline  shown  in  Fig.  2  is  from 
Transbaikalia,  in  Siberia.  The  mineral  takes  its 
name  from  the  celebrated  French  physicist  and  as- 
tronomer, J.  B.  Biot  (1774-1862). 

LEPIDOLITE 

(Plate  34,  Fig.  5). — Lepidolite,  or  lithium-mica, 
is  of  a  characteristic  lilac  or  peach-blossom  color.  It 
usually  occurs  as  compact  scaly  masses,  to  which  the 
cleavages  give  a  spangled  effect.  Such  material,  es- 
pecially that  found  near  Rozena,  in  Moravia,  is  some- 
times cut  and  polished  for  making  small  ornamental 
boxes,  etc.  Lepidolite  invariably  occurs  in  associ- 
ation with  pink  tourmaline  (rubellite),  and  in  Plate 
33>  Fig.  3,  it  is  shown  as  the  matrix  of  this  mineral. 
The  specimen  there  represented  is  from  Pala,  in  San 
Diego  Co.,  California,  where  lepidolite  is  extensively 
mined.  The  mineral  is  used  for  the  extraction  of 
salts  of  lithium,  largely  employed  in  the  manufac- 
ture of  lithia-water. 

ZlNNWALDITE 

(Plate  34,  Fig.  3). — Like  lepidolite,  this  mica  also 
contains  lithium  and  fluorine,  but  in  addition  some 
iron.  In  appearance  it  is  not  unlike  biotite.  It  is 
found  as  crystals — never  of  large  size — in  veins  of 
tin-ore;  the  specimen  represented  in  Fig.  3  being 
from  the  tin-mines  at  Zinnwald,  in  Bohemia. 


SILICATES  (Mica  &  Chlorite  groups). 


Plate  34. 


1,  Muscovite.    2,  Biotite.    3,  Zinnwaldite.    4,  Clinochlore.    5,  Lepidolite. 


THE   CHLORITE   GROUP  239 


THE  CHLORITE  GROUP 

The  chlorites,  or  hydro-micas,  closely  resemble  the 
micas  in  the  form  of  their  scaly  crystals  and  their 
micaceous  cleavage.  The  cleavage  is,  however,  not 
quite  so  perfectly  developed,  and  the  cleavage  flakes, 
though  flexible,  are  not  elastic.  These  minerals  also 
differ  chemically  from  the  micas  in  containing  more 
water  and  no  alkali  metals.  They  are  characteris- 
tically of  a  green  color,  the  name  chlorite  meaning, 
in  Greek,  "green  stone." 

These  minerals  have  usually  been  produced  by  the 
alteration  of  other  minerals,  such  as  biotite,  pyroxene, 
etc.,  and  the  green  color  frequently  shown  by  altered 
igneous  rocks  is  due  to  the  presence  of  chlorites 
amongst  the  products  of  alteration.  They  also  occur 
in  some  schistose  rocks,  and  in  chlorite-schist  (in 
Plate  38,  Fig.  4,  as  the  matrix  of  perovskite)  as  an 
essential  constituent.  Many  varieties  are  distin- 
guished, but  as  a  rule  these  are  only  found  as  com- 
pact masses,  with  a  fine,  scaly  structure.  The  only 
distinctly  crystallized  varieties  are  clinochlore  and 
penninite. 

CLINOCHLORE 

(Plate  34,  Fig.  4). — This  member  of  the  chlorite 
group  is  a  hydrated  silicate  of  aluminium  and  mag- 
nesium, with  variable  amounts  of  iron.  It  is  found 
as  green,  platy  crystals  at  West  Chester,  in  Pennsyl- 
vania (Fig.  4). 


240        THE    WORLD'S    MINERALS 


SERPENTINE 

(Plate  35,  Fig.  i).  —  The  three  minerals  repre- 
sented on  Plate  35  are  all  hydrated  silicates  of 
magnesium,  but,  their  elements  being  combined  in 
different  proportions,  they  are  quite  distinct  chemical 
compounds.  Their  respective  formulae  are: 


Serpentine  ......................... 

Meerschaum  .......................     H4Mg2Si3Oio 

Talc  .....................  .  ........     H,Mg,SuO,, 

A  small  proportion  of  the  magnesium  may  be  re- 
placed by  an  equivalent  amount  of  iron,  this  being 
the  case  more  especially  in  serpentine,  which  usually 
contains  2  or  3  per  cent,  of  ferrous  oxide.  The  water 
contained  in  these  minerals  is  expelled  only  at  a  red- 
heat,  so  that  it  is  present  as  water  of  constitution. 

Serpentine  never  occurs  as  crystals,  but,  neverthe- 
less, it  varies  considerably  in  its  crystalline  structure. 
Usually  it  is  quite  massive  and  compact  (Fig.  i),  but 
sometimes  it  is  lamellar  or  fibrous  ;  the  lamellar  va- 
riety is  known  as  antigorite,  and  the  fibrous  as  chrys- 
otlle.  The  mineral  is  usually  green,  of  a  lighter 
or  darker  shade,  but  is  sometimes  yellowish  or  red. 
A  very  characteristic  feature  is  the  irregular  distri- 
bution of  the  color,  with  mottling  and  veining  like 
that  on  the  skin  of  a  serpent  —  hence  the  name  serpen- 
tine. Specimens  of  a  good  color,  and  with  a  certain 
degree  of  translucency,  are  referred  to  as  noble  or 
precious  serpentine.  Such  material,  and  also  the 


SILICATES. 


Plate  35. 


1,  Serpentine  or  Sepiolite.    2,  Meerschaum.    3,  Talc. 


SERPENTINE  241 

common  variety,  is  extensively  cut  and  polished  for 
ornamental  purposes,  being  frequently  used  in  quite 
large  slabs.  In  its  hardness  (H.  about  3),  and  the  de- 
gree of  polish  it  takes,  it  resembles  marble. 

As  already  mentioned  (p.  219),  rock-masses  of 
serpentine  have  originated  by  the  alteration  in  the 
earth's  crust  of  rocks  composed  largely  of  the  mineral 
olivine.  Large  masses  of  serpentine-rock  are  met 
with  in  many  districts;  for  example,  it  forms  the 
larger  part  of  the  Lizard  peninsula  in  Cornwall. 
Few  visitors  return  from  Lizard  Town  without 
bringing  back  some  of  the  small  ornamental  objects 
carved  in  serpentine.  The  well-known  green  Con- 
nemara  marble  is  a  crystalline  limestone  containing 
much  admixed  serpentine.  The  specimen  represented 
in  Fig.  i  is  from  Reichenstein,  in  Prussian  Silesia. 

The  fibrous  variety,  chrysotile,  occurs  as  veins  in 
the  massive  serpentine-rock.  The  fibers  extend  from 
wall  to  wall  of  the  veins  and  lie  close  together.  In 
mass  this  material  presents  a  very  pretty,  silky  luster, 
with  a  bright  yellowish-green  color ;  but  when  rubbed 
between  the  fingers  it  separates  into  white  cottony 
fibers.  The  veins  vary  from  mere  threads  to  three  or 
four  inches  across,  and  they  form  a  network  through 
the  rock.  Serpentine  is  used  for  the  same  purposes  as 
amphibole-asbestos,  and  is  extensively  mined  in  Can- 
ada. In  the  trade  it  is  known  simply  as  asbestos,  but  to 
avoid  confusion  it  is  best  referred  to  as  serpentine- 
asbestos,  since  the  different  asbestiform  minerals 
differ  so  widely  in  their  chemical  composition  and 
other  essential  characters. 


242         THE    WORLD'S    MINERALS 


MEERSCHAUM 

(Plate  35,  Fig.  2). — The  chemical  formula  of  this 
hydrated  silicate  of  magnesium  is  given  above  on  p. 
240.  It  is  a  compact  mineral,  with  a  fine  earthy 
texture,  and  so  porous  and  light  that,  when  dry,  it 
floats  on  water.  The  name  meerschaum,  in  fact, 
means,  in  German,  "sea-froth."  A  mineralogical 
term  sometimes  employed  is  sepiolite,  which  com- 
pares the  porous  nature  of  the  mineral  to  that  of  a 
cuttle-fish  bone.  Owing  to  its  porosity,  and  conse- 
quently the  avidity  with  which  it  absorbs  water,  the 
mineral  adheres  strongly  to  the  tongue,  even  in  quite 
a  painful  manner  (the  waxed  and  prepared  material 
used  for  tobacco-pipes  does  not,  of  course,  do  this). 
The  material  is  quite  soft,  and  can  be  indented  by 
the  finger-nail. 

Meerschaum  occurs  in  association  with  serpentine- 
rocks;  but  the  material  exported  in  large  quantities 
from  Asia  Minor  is  found  as  small  nodules  in  clayey 
deposits  on  the  plain  of  Eski-Shehr,  where  it  is  dug 
from  numerous  shallow  pits.  To  prepare  the  ma- 
terial for  the  market  it  is  scraped  free  from  dirt  into 
curious  rounded  forms,  dried  in  the  air,  and  the  sur- 
face waxed.  Most  of  the  material  is  exported  through 
Vienna,  and  used  for  the  manufacture  of  tobacco- 
pipes.  Meerschaum  of  inferior  quality  forms  ex- 
tensive beds  at  Vallecas,  in  Spain,  affording  a  light 
but  valuable  building-stone. 


TALC  243 


TALC. 

(Plate  35,  Fig.  3). — The  third  of  these  hydrated 
magnesium  silicates  (see  chemical  formula  on  p.  240) 
differs  from  the  two  preceding  in  being  sometimes 
crystallized ;  never,  however,  as  distinct  crystals,  but 
only  as  foliated  or  lamellar  masses.  These  possess 
a  perfect  micaceous  cleavage  parallel  to  their  sur- 
faces, on  which  the  luster  is  silvery  and  pearly.  Such 
platy  and  scaly  masses  have  a  certain  resemblance 
to  mica,  but  the  cleavage  flakes  are  more  easily  bent, 
and  when  released  they  do  not  spring  back  to  their 
original  position;  that  is,  they  are  flexible,  but  not 
elastic  like  mica.  Further,  the  material  is  greasy  or 
soapy  to  the  touch ;  and,  being  the  softest  of  minerals 
(hardness  No.  i  on  the  scale),  it  is  very  easily 
scratched  with  the  finger-nail.  The  color  is  always 
of  a  delicate  shade  of  apple-green  with  a  silvery  ap- 
pearance (Fig.  3). 

More  frequently,  the  mineral  occurs  as  compact 
masses  with  an  earthy  fracture,  and  a  white  or  greyish 
color.  This  variety  is  known  as  steatite,  and  also 
popularly  as  soap-stone,  French-chalk,  and  pot-stone, 
the  name  talc  being  often  restricted  to  the  foliated 
or  micaceous  variety.  Steatite  finds  many  useful  ap- 
plications. It  is  used  by  tailors  for  marking  cloth; 
for  the  jets  of  gas-burners;  for  furnace-linings,  sink- 
stones,  etc.  In  the  powdered  form,  it  enters  largely 
into  the  composition  of  toilet-powders,  and  is  em- 
ployed for  preserving  rubber,  in  paper-making,  as  a 


244         THE    WORLD'S    MINERALS 

dry  lubricant,  and  for  adulterating  soap.  Being  soft 
and  easily  cut  with  a  knife,  it  is  a  favorite  material 
for  the  grotesque  figures  carved  by  the  Chinese. 

Talc  is  a  very  common  mineral;  it  occurs  in  a> 
sociation  with  serpentine-rocks,  and  also  as  a  constitu- 
ent of  some  schistose  rocks — for  example,  talc-schist 
(shown  in  Plate  26,  Fig.  2,  as  the  matrix  of  the 
needles  of  actinolite).  Extensive  beds  of  steatite  are 
quarried  at  several  places  in  the  United  States,  but 
the  best  qualities  come  from  the  north  of  Italy. 

THE  ZEOLITE  GROUP 

The  several  interesting  minerals  belonging  to  this 
group  are  hydrated  silicates  of  aluminium,  with 
either  sodium  or  calcium,  or  both  of  these.  They  do 
not  form  an  isomorphous  group,  as  do  the  minerals 
of  the  groups  so  far  considered,  for  they  crystallize 
in  several  different  systems.  Water  is  present  in 
relatively  large  amount  (10  to  20  per  cent.),  and  as 
this  is  only  loosely  held  by  the  mineral  it  is  regarded 
as  water  of  crystallization.  When  heated  before  the 
blowpipe  these  minerals  readily  fuse,  and  the  whole 
mass  boils  up  violently,  owing  to  the  rapid  expulsion 
of  the  water.  It  is  on  this  account  that  these  minerals 
receive  the  name  zeolite,  which  means,  in  Greek, 
"boiling  stone." 

The  zeolites  are  minerals  of  secondary  origin,  and 
as  such  occur  in  the  steam-cavities  of  certain  volcanic 
rocks,  more  particularly  those  of  the  basalt  family. 
Their  light  color,  contrasted  with  the  dark  color  of 


ILICATES  (Zeolite  group). 


Plate  36. 


1,  Analcite.    2,  Chabazite. 


THE   ZEOLITE   GROUP  245 

the  rock  in  which  they  are  imprisoned,  makes  them 
conspicuous  objects.  The  matrix  of  volcanic  rock  is 
shown  in  all  the  pictures  in  Plates  36  and  37.  Fur- 
ther, they  are,  as  a  rule,  beautifully  crystallized  with 
brilliant  and  glassy  crystal-faces,  and  often  delicate 
coloring.  Some  of  the  most  attractive  specimens  in 
collections  of  minerals  are  to  be  seen  in  the  showy 
groups  of  crystallized  zeolites.  Being  of  abundant 
occurrence  in  the  volcanic  rocks  of  certain  districts, 
good  specimens  can  be  readily  obtained  by  the  ama- 
teur collector.  The  best-known  localities  are  in  the 
Middle  Lowlands  and  the  Western  Isles  of  Scotland, 
the  north  of  Ireland,  the  Faroe  Islands,  Iceland,  and 
the  Midland  Mountains  in  the  north  of  Bohemia; 
and  there  are  many  other  important  localities. 

Apart  from  the  point  of  view  of  the  collector,  the 
zeolites  present  many  points  of  scientific  interest.  But 
as  far  as  economic  uses  go  they  are  valueless,  except 
that  a  few  massive  varieties  are  used  to  a  very  limited 
extent  as  second-rate  gem-stones.  Nevertheless,  it 
would  appear  that  the  zeolites  play  an  important 
part  in  the  economy  of  nature.  In  a  very  minute  state 
of  division  they  have  been  proved  to  be  present  in 
soils;  and  it  is  probable  that  the  soluble  forms  of  the 
alkalis  necessary  for  the  life  of  plants  are  here  held 
as  constituents  of  zeolites.  The  formation  of  zeolites, 
resulting  from  the  decomposition  of  felspar  and  other 
mineral  particles  in  the  soil,  prevents  the  immediate 
and  total  removal  of  the  alkalis  in  a  still  more  soluble 
form. 

Besides  the  four  species  of  zeolites  figured  on  the 


246         THE    WORLD'S    MINERALS 

accompanying  plates  (36  and  37),  many  others  are 
known  to  mineralogists.  Harmotome  is  of  interest  in 
containing  barium;  bre<wsterite  contains  strontium; 
and  in  phlllipsite  there  is  potassium.  In  laumonlte 
the  water  is  held  so  loosely  that  it  escapes  in  a  dry 
atmosphere,  and  the  transparent  glassy  crystals  crum- 
ble to  powder.  Apophyllite  is  remarkable  for  the 
size  and  perfection  of  its  crystals.  Prehnite,  already 
described  (p.  227),  is  also  sometimes  classified  with 
the  zeolites. 

ANALCITE 

(Plate  36,  Fig.  i). — This  is  a  hydrated  silicate  of 
aluminium  and  sodium.  With  one  rare  exception,  it 
is  the  only  zeolite  that  crystallizes  in  the  cubic  sys- 
tem. The  most  usual  form  is  an  icositetrahedron 
(Fig.  i),  with  the  symbol  (112),  a  twenty-four-faced 
solid  already  mentioned  under  leucite  (p.  209). 
Cubes,  with  each  of  their  corners  replaced  by  three 
small  triangular  faces  of  the  same  form,  are  also  met 
with  in  analcite.  The  walls  of  the  rock-cavities  are 
sometimes  completely  lined  with  a  crust  of  small, 
brilliant,  and  sparkling,  water-clear  crystals;  or, 
again,  a  cavity  may  contain  only  a  few  fine,  large, 
and  opaque  crystals  of  a  white  or  pinkish  color,  as  in 
Fig.  i,  representing  a  specimen  from  the  Fassa  valley 
in  the  Tyrol. 

CHABAZITE 

(Plate  36,  Fig.  2). — In  addition  to  sodium  and  al- 
uminium, this  zeolite  contains  some  calcium.  The 


CHABAZITE— NATROLITE          247 

crystals  are  rhombohedral,  and  the  simple  rhombo- 
hedra  are  very  nearly  cubic  in  their  angles.  Some- 
times, however,  the  form  is  that  of  a  flattened  rhom- 
bohedron,  with  rounded  surfaces,  and  the  crystals  are 
then  very  like  lentil-seeds  in  shape.  Very  often  the 
crystals  are  twinned  with  interpenetration,  the  cor- 
ners of  one  individual  of  the  twin  projecting  through 
the  faces  of  the  other  (Fig.  2).  As  with  analcite, 
crystals  of  chabazite  are  either  colorless  and  trans- 
parent, or  opaque  and  white  or  pinkish.  The  speci- 
men figured,  showing  crystals  in  a  cavity  in  basalt,  is 
from  Nidda,  near  Giessen,  in  Hesse. 

NATROLITE 

(Plate  37,  Figs,  i  and  2). — As  its  name  signifies, 
this  zeolite  contains  sodium,  natrium  being  the  Latin 
name  of  this  element.  The  name  sodalite  (p.  207) 
has  the  same  meaning.  Natrolite  differs,  however, 
from  sodalite  in  containing  water,  but  no  chlorine,  in 
addition  to  soda,  alumina,  and  silica;  and  it  differs 
from  analcite  in  its  constituent  elements  being  com- 
bined in  other  proportions.  The  crystals  are  ortho- 
rhombic,  and  they  often  have  the  form  of  very 
delicate  needles  growing  out  in  thick  clusters  from 
the  surface  of  the  rock.  On  this  account  the  mineral 
was  formerly  known  as  needle-stone.  Frequently  the 
needle-like  crystals  or  fibers  are  closely  aggregated  in 
compact,  radiating  groups,  as  shown  in  Figs,  i  and  2. 
The  isolated  needles  are  usually  perfectly  colorless 
and  transparent;  while  the  radiating  masses  are 


248        THE    WORLD'S    MINERALS 

cloudy  or  opaque,  and  of  a  white,  pinkish,  or  yellow- 
ish color.  The  yellowish  radiating  natrolite  shown 
in  Fig.  2  is  from  a  well-known  occurrence  at  Ho- 
hentwiel,  in  Wiirtemberg;  on  account  of  the  pretty, 
concentric  banding  of  the  color  this  material  is  some- 
times cut  and  polished  for  small  ornaments. 

STILBITE 

(Plate  37,  Fig.  3). — This  contains  calcium,  not 
sodium,  in  addition  to  alumina,  silica,  and  water. 
The  crystals  are  monoclinic,  but  owing  to  twinning 
they  are  very  complex.  Sometimes  they  have  the 
form  of  elongated  six-sided  plates;  but  more  often  the 
crystals  are  grouped  in  sheaf -like  aggregates  (Fig.  3, 
from  Iceland),  this  form  being  a  very  characteristic 
feature  of  the  mineral.  The  crystals  have  a  perfect 
cleavage  in  one  direction,  and  on  this  surface  the 
luster  is  markedly  pearly  in  character.  Although  the 
mineral  is  usually  snow-white,  crystals  from  Old 
Kilpatrick,  in  Dumbartonshire,  are  brick-red  in 
color. 


SILICATES  (Zeolite  group). 


Plate  37. 


1,  2,  Natrolite.    3,  Stilbite. 


CHAPTER  XIII 

THE  TITANO-SILICATES,  TITANATES, 
AND  NIOBATES 

THE  preceding  chapter  on  the  silicates  is  by  far  the 
longest  in  the  book;  the  present  one  will  be  the 
shortest.  The  rare  minerals  here  grouped  together 
are  salts  of  the  metallic  acids,  titanic  acid  and  niobic 
acid.  Titanic  acid  is  allied,  chemically,  to  silicic 
acid,  and  in  the  mineral  sphene  both  of  them  are 
present.  On  the  other  hand,  niobic  acid  is  related  to 
arsenic  and  vanadic  acids.  In  a  strictly  chemical 
classification,  therefore,  the  titanates  should  be  placed 
with  the  silicates,  and  the  nibates  with  the  phosphates 
and  arsenates.  But  as  a  matter  of  convenience,  the 
titano-silicates  and  titanates  may  be  considered  as  an 
appendix  to  the  silicates;  and  since  niobic  and  titanic 
acids  are  often  present  together  in  the  same  minerals, 
there  is  some  excuse  for  grouping  the  niobates  with 
the  titanates. 

SPHENE 

(Plate  38,  Figs,  i  and  2). — Though  not  an  abun- 
dant mineral,  sphene  is  by  far  the  commonest  of  this 
group,  the  others  being  of  quite  rare  occurrence.  It 
is  a  silicate  and  titanate  of  calcium,  with  the  formula 
CaTiSiOs.  Since  it  contains  the  element  titanium,  it 

249 


250        THE   WORLD'S    MINERALS    . 

is  often  known  as  titanite.  The  name  sphene  refers  to 
the  wedge-shaped  form  of  the  monoclinic  crystals. 
Crystals  of  the  size  and  perfection  of  those  shown  in 
the  accompanying  pictures  are  most  exceptional. 
Usually  they  are  quite  small,  and  are  found  embed- 
ded in  igneous  and  crystalline  rocks  of  various  kinds 
— for  example,  in  the  granite  of  Criffel,  in  Galloway. 
The  best  crystals,  of  a  green  or  greenish-yellow  color 
and  often  transparent,  are  found  on  the  walls  of 
crevices  in  the  gneissic  rocks  of  the  Alps  (Fig.  i) ; 
while  the  largest,  of  a  dark-brown  color,  are  met  with 
in  crystalline  limestones  in  Canada  and  the  United 
States,  the  specimen  shown  in  Fig.  2  being  from 
Diana,  in  Lewis  Co.,  New  York. 

Transparent  crystals  are  occasionally  cut  as  gems. 
These  display  pretty  green  and  red  flashes  of  color; 
but  unfortunately  they  are  rather  too  soft  (H.  =  5^ ) 
to  withstand  much  wear. 

PEROVSKITE 

(Plate  38,  Fig.  4). — This  is  a  titanate  of  calcium, 
CaTiO3,  with  a  formula  analogous  to  that  of  the 
calcium  silicate  wollastonlte;  but  there  is  no  further 
similarity  between  these  two  minerals.  Perovskite  is 
found  as  dull  or  lustrous,  dark-brown  or  black  cubes 
in  chlorite-schist  at  Zermatt,  in  Switzerland,  and  at 
Achmatovsk,  in  the  Ural  Mountains.  The  specimen 
represented  in  Fig.  4  is  from  the  latter  locality  (the 
white  mineral  here  shown  is  calcite).  The  crystals, 
though  presenting  the  external  form  of  cubes,  have  a 


TITANO-SILICATES,  NIOBATES,  &c. 


Plate  38. 


1,  2,  Sphene.    3,  Columbite.    4,  Perovskite. 


f    1 


0   T   |  J  f    G 


COLUMBITE  251 


complex  internal  structure  similar  to  that  shown  by 
leucite. 

COLUMBITE 

(Plate  38,  Fig.  3). — This  rare  species  is  a  niobate 
of  iron,  FeNb2O6,  but  the  iron  is  often  partly  re- 
placed by  manganese,  and  the  niobium  by  tantalum. 
The  metal  columbium  (later  renamed  niobium)  was 
discovered  in  this  mineral  by  the  English  chemist  and 
mineralogist,  Charles  Hatchett,  in  the  year  1802,  and 
was  so  named  by  him  because  the  specimen  he  ex- 
amined came  from  America;  the  mineral  itself  he 
called  ore  of  columbium,  and  very  soon  afterwards  it 
wras  given  the  mineralogical  name  columbite.  The 
mineral  occurs  as  well-developed  orthorhombic  crys- 
tals in  pegmatite-veins.  In  color  they  are  dark  brown 
or  black,  usually  with  dull  faces,  but  sometimes  they 
are  very  lustrous.  The  specimen  represented  in  Fig. 
3,  with  brilliant,  black  crystals  embedded  in  white 
quartz,  is  from  Standish,  in  Maine. 

When  the  amount  of  tantalum  exceeds  that  of  ni- 
obium we  have  the  very  similar  and  isomorphous 
mineral  tantalite,  which  is  thus  essentially  a  tantalate 
of  iron,  FeTa2O6.  Within  the  last  few  years  this 
rare  mineral  has  come  to  be  of  considerable  impor- 
tance as  a  source  of  the  heavy  metal  tantalum,  now 
extensively  used  for  the  metallic  filaments  of  incan- 
descent electric  lights.  Being  a  metal  possessed  of 
great  hardness,  and  other  exceptional  properties,  it 
will  no  doubt  in  the  future  find  many  other  useful 
applications. 


CHAPTER  XIV 

THE  ORGANIC  SUBSTANCES 

THE  substances  to  be  here  considered  cannot,  strictly 
speaking,  be  regarded  as  mineral  species,  for,  with 
very  few  exceptions,  they  possess  neither  a  crystalline 
structure  nor  a  definite  chemical  composition.  Many 
of  them  are  clearly  of  organic  origin,  and  represent 
the  fossilized  remains  of  plants;  but  this  is  not  always 
the  case,  as  will  be  mentioned  under  asphaltum.  In 
the  systems  of  classification  in  mineralogy  they  are 
usually  relegated  to  an  appendix  or  omitted  alto- 
gether. But  on  account  of  their  great  economic  im- 
portance they  cannot  be  completely  neglected. 

Chemically,  they  consist  of  carbon  in  combination 
with  hydrogen  or  with  hydrogen  and  oxygen,  and 
they  are  therefore  described  as  hydrocarbons.  It  is, 
however,  important  to  remember  that  some  of  the 
minerals  we  have  already  described  as  definite  species 
also  contain  carbon — for  example,  the  carbonates 
(Chapter  IX) — while  diamond  and  graphite  consist 
wholly  of  this  element.  In  addition  to  these  there 
are  also  a  few  other  rare  minerals,  occurring  in  a 
crystallized  form,  which  consist  of  salts  of  the  so- 
called  organic  acids — for  example,  the  mineral  <whe- 
wellite,  a  hydrated  oxalate  of  calcium. 

252 


AMBER  253 


AMBER 

(Plate  39,  Fig.  i). — Amber  is  a  fossil  resin  found 
as  small  nodules  in  certain  beds  of  clay  and  sand. 
These  nodules,  which  occasionally  have  the  form  of 
drops  or  tears,  represent  the  drops  of  viscous  resin 
exuded  from  the  trees  of  a  species  of  pine,  Pinites 
succinifer,  now  long  since  extinct,  which  in  past  ages 
of  the  earth's  history  (the  Oligocene  period  of  geolo- 
gists) grew  on  the  site  now  occupied  by  the  Baltic 
and  the  North  Sea  and  a  portion  of  northern  Europe. 
The  resin  no  doubt  became  buried  in  the  soil,  in  the 
same  way  that  we  find  at  the  present  day  the  kauri- 
gum  in  New  Zealand  and  the  copal-resin  near  Zan- 
zibar. When,  however,  owing  to  movements  of  the 
earth's  crust,  these  forest  areas  came  to  be  submerged 
below  sea-level,  the  surface  materials  with  their  con- 
tained amber  were  re-sorted  and  deposited  on  the 
bottom  of  the  sea  as  beds  of  sand  and  clay.  Here 
the  resin  remained  for  countless  ages,  undergoing  a 
slow  hardening  and  change  in  the  nature  of  its  sub- 
stance. 

Much  of  this  history  can  be  made  out  from  a  study 
of  the  various  substances  which  we  find  enclosed  in 
amber,  such  as  fragments  of  wood,  pine-needles, 
leaves,  insects  of  various  kinds,  and  even  snails  and 
other  small  animals,  inhabitants  of  the  primeval  for- 
est, which  got  caught  in  the  viscous  resin.  Fig.  i  of 
Plate  39  shows  an  insect  enclosed  in  a  cut  and  pol- 
ished piece  of  Baltic  amber. 


254         THE.  WORLD'S    MINERALS 

The  amber-bearing  beds  now  outcrop  in  the  north 
of  Prussia,  and  they  also  form  the  floor  of  a  portion 
of  the  Baltic.  When  these  submarine  strata  are  dis- 
turbed by  storms,  the  nodules  of  amber  are  washed 
out  and  cast  up  on  the  beach.  Formerly,  nearly  all 
the  amber  collected  was  picked  up  off  the  beach  or 
netted  in  the  shallow  water.  Later,  dredging  was 
resorted  to;  but  at  the  present  time  the  largest  quan- 
tities of  amber  are  obtained  by  a  regular  system  of 
underground  mining  in  the  amber-bearing  strata. 
These  strata  extend  to  the  east  into  Russia,  and  west- 
ward into  Denmark;  while  occasionally  fragments 
of  amber  are  picked  up  on  the  coasts  of  Essex,  Suf- 
folk, and  Norfolk.  The  richest  deposits  are,  how- 
ever, those  in  the  northeast  of  Prussia,  in  the  district 
of  Samland,  and  practically  the  whole  of  the  amber 
trade  is  centered  round  Danzig  and  Konigsberg. 
This  amber  is  therefore  known  as  Prussian  or  Baltic 
amber  to  distinguish  it  from  other  less  valuable  fossil 
resins  of  somewhat  similar  character  found  in  other 
parts  of  the  world. 

As  is  well  known,  amber  is  yellow  in  color — in- 
deed, we  speak  of  amber-yellow,  but  the  shade  of 
yellow  differs  widely  in  different  specimens;  it  may 
be  very  pale,  almost  white,  to  a  deep  reddish-yellow 
or  brown.  Also  in  the  degree  of  transparency  there 
is  a  wide  range;  the  best  pieces  are  perfectly  trans- 
parent and  clear  (as  in  Fig.  i);  while  others  are 
cloudy  and  opaque,  these  being  known  as  bone  or 
osseous  amber.  Sometimes  there  are  both  clear  and 
cloudy  patches  or  bands  in  the  same  piece.  The 


ORGANIC  SUBSTANCES. 


Plate  39. 


U  k  I  V  .    01- 
L,  LI  1-0  «.v  U  I  £ 


1,  Amber  (enclosing  insect).    2,  Asphaltum.    3,  Lignite. 


i  K  f  I 


/•  i 


AMBER— ASPHALTUM  255 

opacity  is  due  to  the  presence  of  vast  numbers  of 
microscopic  air-bubbles  in  the  amber.  The  specific 
gravity  is  1.05;  that  is,  amber  is  only  slightly  heavier 
than  water.  Its  hardness  is  2*/>  on  the  mineralogist's 
scale,  being  rather  greater  than  that  of  gypsum  and 
alabaster.  Chemically,  it  consists  of  carbon,  hydro- 
gen, and  oxygen  in  approximately  the  proportions 
given  by  the  formula  CioHieO.  As  shown  by  the 
action  of  various  solvents  (turpentine,  alcohol,  etc.), 
it  really  consists  of  a  mixture  of  resins;  and,  in  addi- 
tion, it  contains  an  organic  acid  known  as  succinic 
acid,  the  presence  of  which  is  an  important  character 
of  the  true  Baltic  amber  (succinite). 

Being  a  bad  conductor  of  electricity,  amber  be- 
comes electrified  when  rubbed  on  cloth,  and  it  will 
then  attract  to  itself  small  bits  of  paper  and  other 
light  objects.  It  is  from  the  Greek  name  elektron  for 
amber  that  our  word  electricity  is,  on  this  account, 
derived. 

The  uses  of  amber  are  well  known.  It  is  cut  and 
polished  as  beads  and  other  personal  ornaments,  and 
for  the  mouth-pieces  of  tobacco-pipes,  cigar-  and 
cigarette-holders.  Small  fragments  and  chips  when 
heated  under  hydraulic  pressure  become  welded  to- 
gether, and  the  pressed  amber,  or  ambroid,  so  ob- 
tained is  put  to  the  same  purposes  as  the  natural 
pieces. 

ASPHALTUM 

(Plate  39,  Fig.  2). — Asphalt,  bitumen,  or  mineral- 
pitch  is  a  jet-black  substance,  breaking  with  a  smooth 


256         THE    WORLD'S    MINERALS 

and  bright  conchoidal  fracture  (Fig.  2).  It  varies, 
however,  considerably  in  its  characters,  and  is  really 
an  indefinite  mixture  of  hydrocarbons,  which  are  in 
part  oxygenated.  The  specimen  represented  in  the 
picture  is  from  the  famous  pitch-lake  on  the  island 
of  Trinidad.  This  lake,  or  rather  cake,  of  asphaltum 
has  a  circumference  of  one  and  a  half  miles,  and  a 
depth  of  eighteen  to  seventy-eight  feet.  It  is  said 
that  formerly  the  material  was  liquid  and  warm  to- 
wards the  center,  but  now  it  is  solid  throughout.  A 
viscous  bitumen  oozes  from  the  ground  round  the 
Dead  Sea,  while  in  the  lake  itself  there  occasionally 
rise  to  the  surface  considerable  masses  of  bituminous 
matter.  As  this  dries  on  exposure  to  the  sun  it  sets 
as  asphaltum.  This  material  was  used  by  the  ancients 
for  pitching  their  boats,  and  the  Greek  name  Lake 
Asphaltites,  meaning  "bituminous  lake,"  for  the 
Dead  Sea,  is  the  origin  of  our  name,  asphaltum. 

Asphaltum  impregnates  many  rocks  of  sedimentary 
origin;  but  it  also  sometimes  occurs  in  small  amounts 
in  igneous  and  crystalline  rocks  and  in  some  mineral- 
veins.  In  the  latter  case  the  substance  can  scarcely 
be  of  organic  origin.  In  some  of  the  Derbyshire 
lead-mines  there  is  found  a  peculiar  form  of  as- 
phaltum known  as  elaterite,  or  elastic  bitumen,  which 
is  soft  and  elastic  like  india-rubber. 

LIGNITE 

(Plate  39,  Fig.  3). — This  is  merely  fossil  wood, 
still  plainly  showing  the  woody  structure  (Fig.  3). 


LIGNITE— COAL  257 

It  has  thus  little  claim  to  rank  as  a  mineral  in  our 
meaning  of  the  term.  The  degree  of  the  f  ossilization 
may,  of  course,  vary  between  wide  limits,  for  lignite 
is  only  an  intermediate  stage  in  the  formation  of  coal. 
Extensive  beds  of  lignite  are  found  in  strata  of  the 
more  recent  (Tertiary)  geological  periods — for  ex- 
ample, at  Bovey  Tracey,  in  Devonshire.  The  speci- 
men represented  in  the  picture  is  from  Germany,  in 
which  country  lignite,  or  brown  coal,  is  of  consider- 
able importance  as  a  fuel. 

In  strata  of  Liassic  age  near  Whitby,  in  Yorkshire, 
there  is  found  a  still  more  fossilized  variey,  approach- 
ing coal  in  character;  this  is  black  and  compact, 
though  the  exterior  form  of  the  masses  is  that  of 
tree-trunks.  This  is  the  material  well  known  as  jet, 
which  is  extensively  cut  and  polished  for  a  variety 
of  small  ornaments. 

COAL 

(Plate  40,  Fig.  i). — It  would  be  out  of  place  here 
to  give  an  adequate  account  of  the  characters,  occur- 
rence, and  origin  of  this  important  substance,  on 
which  the  industrial  prosperity  of  the  present  age  so 
largely  depends.  It  is  found  as  beds  or  seams,  vary- 
ing in  thickness  from  a  fraction  of  an  inch  to  thirty 
feet  or  more,  interbedded  with  sandstones  and  shales 
in  the  rocks  of  various  geological  periods,  but  mainly 
in  those  of  the  period  to  which  the  name  carbon- 
iferous is  appropriately  applied.  At  this  remote 
period  of  the  world's  history  there  were  extensive 
swamps,  with  luxurious  growths  of  vegetation — vege- 


258        THE    WORLD'S    MINERALS 

tation  of  quite  a  different  character  from  that  of  the 
present  day.  Thick  accumulations  of  vegetable  mat- 
ter were  formed  in  much  the  same  way  that  peat 
forms  at  the  present  day;  and  these  became  covered 
by  the  sedimentation  of  clay  and  sand,  and  so  pre- 
served. With  a  slow  and  continual  sinking  of  the 
area,  owing  to  movements  of  the  earth's  crust,  a  great 
thickness  of  alternate  layers  of  sand,  clay,  and  vege- 
table remains  was  piled  up.  Under  this  enormous 
pressure,  and  with  various  chemical  reactions  in  the 
presence  of  water,  acting  for  millions  of  years,  the 
sands  have  become  converted  into  sandstones,  the 
clays  into  shales,  and  the  vegetable  remains  into  coal. 
Trunks  and  roots  of  fossil  trees  are  frequently  found 
in  the  rocks  of  the  coal-measures;  but  in  the  coal 
itself  the  material  has  been  so  altered  by  fossilization 
that  little  organized  structure  remains,  and,  as  a  rule, 
can  only  be  detected  when  thin  sections  of  the  ma- 
terial are  examined  under  the  microscope. 

The  reader  will  no  doubt  have  noticed  that  some 
kinds  of  coal  break  into  small  cube-like  blocks,  with 
smooth  and  bright  surfaces.  This  fracture  is  a  result 
of  jointing  in  the  material,  and  is  altogether  distinct 
from  the  cleavage  of  crystals,  since,  although  the 
blocks  are  sometimes  quite  small,  the  sub-division 
cannot  be  repeated  indefinitely. 

Often,  also,  it  may  be  noticed  that  such  surfaces 
have  a  thin,  brassy  coating,  looking  like  gold.  This 
is  the  mineral  iron-pyrites,  which  is  sometimes  pres- 
ent as  small  nodules,  causing  the  coal  to  detonate  and 
fly  about  when  placed  in  the  fire. 


ORGANIC  SUBSTANCES. 


Plate  40. 


•1,  Coal.    2,  Anthracite, 


ANTHRACITE  259 


ANTHRACITE 

(Plate  40,  Fig.  2). — This  is  a  variety  of  coal  in 
which  the  process  of  fossilization  has  proceeded  still 
further,  and  no  trace  of  organized  structure  remains. 
The  volatile  material  is  much  reduced  in  amount, 
and  the  material  consists  largely  (up  to  95  per  cent.) 
of  pure  carbon.  Anthracite  is  brittle,  and  breaks 
with  a  smooth  and  bright  conchoidal  fracture  very 
like  pitch,  as  may  be  seen  from  a  comparison  of  the 
two  pictures  on  Plates  39  and  40.  The  surfaces  of 
anthracite  sometimes  display  a  brilliant  iridescent 
tarnish.  The  chief  sources  of  supply  are  the  coal- 
fields of  South  Wales  and  Pennsylvania. 


INDEX 


Abrasives,  52. 
Achroite,  229. 
Acicular  crystals,  26. 
Actinolite,  198. 
Acute  pyramid,  19. 

rhombohedron,  24. 

Adamantine  luster,  38,  56. 

Adamas,  55. 

Adularia,  190. 

Agate,  117. 

Aggregation  of  crystals,  29. 

Alabaster,  165. 

Albite,  192. 

twin-law,  187. 

Alluvial  deposits,  57,  73,  130. 
Almandine,  213,  215. 
Alum  crystals,  6,  16. 
Aluminium,  126,  127. 

oxide,  125. 

phosphates,  177,  178,  181. 

silicates,  223,  &c. 

Amalgam,  73. 
Amazon-stone,  192. 
Amber,  2,  253. 
Ambroid,  255. 
Amethyst,  115. 

oriental  (corundum),  35. 

Amorphous  minerals,  32. 
Amphibole  group,  195. 
Amphibole-asbestos,  199. 
Analcite,  246. 
Analysis,  chemical,  48. 
Anatase,  49. 
Andalusite,  223. 
Andradite,  213,  216. 
Angles  of  crystals,  15,  20,  113. 


261 


Angles,  re-entrant,  28. 
Anglesite,  161. 
Annabergite,  177,  97. 
Anorthic  system,  8,  23. 
Anorthite,  195. 
Anthracite,  259. 
Antigorite,  240. 
Antimonite,  79. 
Antimony-glance,  79. 

• ,  native,  68. 

• ores,  52. 

• sulphide,  78. 

Apatite,  171. 

Apophyllite,  246. 

Aquamarine,  211. 

Aragonite,  146. 

Arizona  "  ruby  "  (garnet),  216. 

Arsenates,  170. 

Arsenic,  native,  58. 

ores,  52. 

sulphides,  80,  81. 

,  white,  90. 

Arsenical  pyrites,  89. 
Arsenides,  78. 
Arsenopyrite,  89. 

Artificial   corundum    (ruby,  &c.), 
128. 

diamond,  60. 

Asbestiform  minerals,  241. 
Asbestos,  1 08,  202,  241. 
Aschtrekker,  231. 
Asphalt,  255. 
Asphaltum,  255. 
Atacamite,  108. 
Augite,  204. 
Augite-syenite,  190. 


262 


INDEX 


Auripigmentum,  81. 
Australian  opal,  120. 
Autunite,  179. 
Avanturine  felspar,  193. 
Axes,  crystallographic,  9. 

of  reference,  9. 

of  symmetry,  u. 

,  twin-,  28. 

Axinite,  227. 
Azurite,  153. 

Balas-ruby,  125. 

Ball- jasper,  117. 

Baltic  amber,  253. 

Banket,  74. 

Barium  sulphate,  157. 

Barytes,  157. 

Basal  pinacoid,  20,  25. 

plane,  20. 

Basalt,  3. 

Batea,  57. 

Bauxite,  109. 

Beryl,  211. 

Biotite,  237. 

Bipyramid,  19,  21. 

Bismuth,  native,  69. 

Bismuthinite,  70. 

Bitumen,  255. 

Black-band  iron-stone,  145. 

Black-jack,  84. 

Black  lead,  64. 

Blende,  83. 

Bloodstone,  117,  122. 

Blue  asbestos,  202. 

ground,  53. 

Blue-iron-earth,  181. 
Blue-John,  104,  105. 
Bog-iron-ore,  134. 
Bohemian  garnet,  216. 
Bone  amber,  254. 
Books  of  mica,  234. 
Bort,  61. 

Botryoidal  forms,  31,  227. 
Brachy-dome,  160. 


Brazilian     emerald    (tourmaline), 
229. 

• pebble,  118. 

Brewsterite,  246. 

Brilliant  form  of  cutting,  60. 

Brittleness,  55. 

Broggerite,  133. 

Bromoform,  40. 

Bronzite,  205. 

Brookite,  49. 

Brown  coal,  257. 

Brown-iron-ore,  134. 

Brown-lead-ore,  174. 

Bruting,  60. 

Burnt  topaz,  221. 

Cabochon,  215. 

Cairngorm,  115. 

Calamine,  143. 

Calcite,  139. 

Calcium  carbonates,  139,  146. 

fluoride,  102. 

phosphate,  171. 

silicate,  206. 

sulphate,  162. 

titanate,  250. 

tungstate,  170. 

Calcouranite,  179. 
Calc-spar,  139. 
Californite,  220. 
Cameos,  118. 
Campylite,  175. 
Cannon-spar,  140. 
Cape  ruby  (garnet),  216. 
Capillary  crystals,  26. 
Carat,  61,  73- 
Carbon,  53. 

compounds,  252. 

Carbonado,  61. 
Carbonates,  138. 
Carbonic  acid,  138. 
Carborundum,  127. 
Carbuncle,  215. 
Carnelian,  117. 


INDEX 


263 


Cassiterite,  129. 

Cat's-eye,  115. 

Celestite,  160. 

Celts,  202. 

Center  of  symmetry,  12,  23. 

Cerargyrite,  109. 

Cerusite,  150. 

Chabazite,  246. 

Chalcedony,  117. 

Chalcopyrite,  95. 

Chalcotrichite,  136. 

Chalk,  139. 

Chalybite,  144. 

Characters  of  minerals,  5. 
Chatoyancy,  116. 
Chemical  analysis,  48. 

characters,  46. 

compounds,  47. 

elements,  I,  46,  53. 

formula,  48. 

Chessylite,  152. 

Chiastolite,  224. 

China-clay,  189. 

Chloanthite,  97. 

Chlor-apatite,  171. 

Chlorides,  101. 

Chlorite  group,  239. 

Chromates,  156. 

Chrome-iron-ore,  125. 

Chromite,  125. 

Chrysolite,  219. 

Chrysoprase,  117. 

Chrysotile,  240. 

Cinnabar,  88. 

Cinnamon-stone,  215. 

Citrine,  115. 

Classification  of  minerals,  50,  185. 

Clay,  189. 

Clay-iron-stone,  145. 

Cleavage,  41. 

Cleopatra's  emerald  mines,  212. 

Cleveite,  133. 

Clinochlore,  239. 

Clino-pinacoid,  164. 


Coal,  257. 

Cobalt  arsenide,  97. 

arsenate,  176. 

ores,  52. 

Cobalt-bloom,  97,  176. 
Cobalt-blue,  98. 
Cockscomb-pyrites,  91. 
Cohesion  of  Crystals,  42. 
Colloids,  32. 
Color,  range  of,  35. 

,  zones  of,  231. 

Color-varieties,  35. 
Coloring  matters,  35. 
Colors  of  corundum,  126. 

of  fluor-spar,  103. 

of  gem-stones,  126. 

of  minerals,  34. 

,  interference-,  37,   120,  194. 

,  iridescent,  37,  96. 

Columbite,  251. 
Columnar  crystals,  25. 

structure,  30. 

Combination  of  forms,  14,  20. 
Common  felspar,  189. 

hornblende,  196. 

opal,  120. 

Compact  aggregates,  30,  116. 
Concentric  shelly  structure,  31. 
Conchoidal  fracture,  43. 
Connemara  marble,  241. 
Constancy  of  angles,  15. 
Copal-resin,  253. 
Copper  carbonates,  152,  154. 

,  native,  75. 

ores,  51. 

oxide,  136. 

oxychloride,  108. 

sulph-antimonite,  98. 

sulphide,  95. 

uranium  phosphate,  179. 

Copper-nickel,  87. 
Copper-pyrites,  95. 
Coral,  2. 
Coxalloidal  forms,  31. 


264 


INDEX 


Cordierite,  37. 
Cornelian  =  carnelian,  1 17. 
Corundum,  35,  126. 
Crocidolite,  199,  201. 

(quartz),  116. 

Crocoite,  166. 

Cross-hatched  structure,  191. 
Cross-stone,  224,  232. 
Cryolite,  109. 

Crypto-crystalline,  III,  116. 
Crystal  balls,  119. 

models,  10. 

systems,  7. 

Crystalline  grains,  29. 

individuals,  29. 

Crystallization,  6,  29. 

,  water  of,  244. 

Crystallographic  axes,  8. 
Crystallography,  7. 
Crystals,  6. 

,  aggregation  of,  29. 

,  growth  of,  6. 

,  habit  of,  26. 

,  intergrowths  of,  27. 

,  malformation  of,  16. 

,  shapes  of,  7. 

Cube,  9,  18. 
Cubic  cleavage,  42. 

habit,  27. 

system,  8,  9,  19. 

Cubo-octahedron,  15. 
Cullinan  diamond,  62. 
Cuprite,  136. 
Cuprouranite,  178. 
Cutting  of  gem-stones,  60. 
Cyanide  process,  74. 
Cyanite,  225. 

Danaite,  90. 

Demantoid,  217. 

Dendritic  forms,  31,  118. 

Density,  38. 

Destruction  of  crystals,  32.   . 

Diamond,  53. 


Diatomite,  121. 

Dichroism,  37,  231. 

Dichroite,  37. 

Dichroscope,  37. 

Dimorphism,  49,  91,  146,  191,  210, 

231. 

Diopside,  202. 
Directional  characters,  34. 
Dispersive  power,  56. 
Distortion  of  crystals,  16. 
Dog-tooth-spar,  140. 
Dolomite,  143. 
Dome  faces,  90. 
Double  refraction,  142. 
Doubly-refracting  spar,  142. 
Dravite,  229. 
Drusy  surfaces,  114. 
Dunite,  218. 
Dyad  axis,  12. 
Dyes  of  minerals,  35. 

Earthy  odor,  45. 
Edges,  parallelism  of,  17. 
Eisenkiesel,  36. 
Elastic  bitumen,  256. 
Elaterite,  256. 
Electrical  properties,  44. 
Elektron,  255. 
Elements,  chemical,  46,  53. 
Elie  "  ruby  "  (garnet),  216. 
Emerald,  211. 

,  Brazilian  (tourmaline),  229. 

,  oriental  (corundum),  35. 

,  Uralian  (garnet),  217. 

Emery,  127. 
En  cabochon,  215. 
Enstatite,  205. 
Epidote,  226. 
Erythrite,  175,  97. 
Essonite  =  hessonite,  215. 
Excelsior  diamond,  62. 
External  characters,  50. 

Faces  of  crystals,  9. 


INDEX 


265 


Faces,  symbols  of,  13. 
Faceted  gems,  60. 
Fahlerz,  98. 
Fahlore,  98. 
False  lead,  84. 

topaz,  115. 

Fayalite,  218. 
Felspar  group,  186. 
Felt-like  aggregates,  30. 
Fertilizers,  170. 
Fibers,  crystal,  30. 
Fibrous  structure,  30. 
Fire  of  diamond,  56. 
First  water  diamond,  57. 
Flint,  no. 

Florentine  diamond,  61. 
Flos-ferri,  148. 
Flowers  of  sulphur,  67. 
Fluor,  101. 
Fluor-apatite,  171. 
Fluorescence,  104. 
Fluorides,  101. 
Fluorite,  101. 
Fluor-spar,  101. 
Flux,  1 02. 
Forms,  crystal,  14. 

of  aggregation,  29. 

of  cutting  for  gems,  60. 

of  minerals,  6. 

Formula,  chemical,  48. 
Forsterite,  218. 
Fossil  resin,  253. 

wood,  256. 

Fracture,  41. 
Frangibility,  55. 
French-chalk,  243. 
Fuchsite,  236. 

Gahnite,  125. 
Galena,  86. 
Garnet  group,  213. 
Garnetohedron,  214. 
Gaseous  elements,  47,  53. 
Gem-stones,  37,  41,  52. 


Gems,  famous  diamonds,  61. 

Geology,  i. 

Geometrical  regularity,  16. 

solids,  6. 

Geyserite,  121. 
Glassy  felspar,  190. 

luster,  38. 

• minerals,  32. 

Glazier's  diamond,  61. 
Globular  forms,  31. 
Gold,  native,  72. 

quartz,  74. 

• tellurides,  74. 

Gooseberry-stone,  218. 
Granite,  3,  189. 
Granular  habit,  214. 

structure,  29. 

Graphite,  63. 
Greasy  feel,  45. 

luster,  38. 

Green-lead-ore,  174. 
Greenstone,  New  Zealand,  201. 
Grey-copper-ore,  98. 
Grossularite,  213,  218. 
Growth  of  crystals,  6,  16,  27,  32. 
Gypsum,  162. 

Habit  of  crystals,  26. 
Hackly  fracture,  43. 
Haematite,  121. 
Hair-copper,  136. 
Hair-like  crystals,  26. 
Hair-pyrites,  26. 
Halite,  106. 
Halogens,  101. 
Haloids,  101. 
Hardness,  43. 

of  diamond,  55,  61. 

of  kyanite,  225. 

Harmotome,  246. 
Heaviness,  38. 
Heavy  liquids,  39. 

spar,  158. 

Heliotrope,  117. 


266 


INDEX 


Hematite  =  haematite,  121. 
Hemimorphic  crystals,  229. 
Hemi-pyramid,  23. 
Hemispherical  forms,  31. 
Hessonite,  215. 
Hexagonal  prism,  25. 

•  system,  8,  25. 

Hiddenite,  207. 
Hornblende,  196. 
Horn-silver,  109. 
Hornstone,  117. 
Hungarian  opal,  121. 
Hyacinth,  131. 
Hyalite,  119. 
Hydrocarbons,  252. 
Hydro-micas,  239. 
Hypersthene,  205. 

Iceland-spar,  141. 
Ice-spar,  109,  190. 
Icositetrahedron,  14,  18,  209. 
Idocrase,  219. 
Ilmenite,  123. 
Index  of  refraction,  56. 
Indian  diamonds,  61. 
Indicolite,  229. 
Inflammables,  52. 
Infusorial  earth,  121. 
Inorganic  bodies,  i,  46. 
Intercepts  on  axes,  13. 
Interference-colors,  37,  120,  194. 
Intergrowths  of  crystals,  27. 
Iridescent  colors,  37,  96. 
Iron  carbonate,  144. 

hydroxide,  134. 

,  native,  76. 

niobate,  251. 

ores,  36,  51. 

oxides,  121,  123. 

phosphate,  180. 

silicates,  202,  205,  218,  &c. 

sulph-arsenide,  89. 

sulphides,  91,  92,  94. 

tungstate,  167. 


Iron-glance,  121. 
Iron-mica,  235. 
Iron-pyrites,  92,  32,  48. 

in  coal,  258. 

Irregular  intergrowths,  27. 
Irregularity  of  crystals,  15. 
Isodimorphism,  151. 
Isomorphism,  49. 

Isomorphous  series,    143,  185,  213, 
&c. 

Jade,  200. 
Jadeite,  206. 
Jargoon,  131. 
Jasper,  116. 

Jet,  257- 

Jeweler's  rouge,  123. 

Kaolin,  189. 
Kauri-gum,  253. 
Kidney-iron-ore,  122. 
Kidney-stone,  201. 
Kieselguhr,  121. 
Kinds  of  minerals,  3. 
King's  yellow,  81. 
Knee-shaped  twins,  28. 
Koh-i-noor  diamond,  62. 
Krystallos,  115. 
Kunzite,  207. 
Kupfernickel,  87. 
Kyanite,  225. 

Labradorite,  193. 
Labrador-spar,  193. 
Lake-ore,  134. 
Lamellar  twinning,  188. 
Lapis-lazuli,  208. 

nephriticus,  201. 

Laumontite,  246. 
Lazulite,  178. 
Lazurite,  208. 
Lead  arsenate,  173. 

carbonate,  150. 

chromate,  166. 


INDEX 


267 


Lead  molybdate,  168. 

ores,  51. 

phosphate,  174. 

sulphate,  161. 

sulphide,  86. 

vanadate,  175. 

Leaf-gold,  72. 

Leafy  forms,  31. 

Left-handed  crystals,  112. 

Lepidolite,  238. 

Lepidomelane,  235. 

Leucite,  209. 

Light  and  crystals,  34,  38. 

,  polarized,  231. 

Lignite,  256. 
Lime-felspar,  186,  195. 
Limestone,  138,  208. 
Limonite,  32,  133. 
Linarite,  166. 
Liquids,  heavy,  39. 
Lithium-mica,  238. 
Lodestone,  124. 
Luster,  37. 

Macro-axis,  157. 
Macro-dome,  157. 
Magnesite,  143. 
Magnesium  carbonate,  143. 

silicates,  205,  218,  240,  &c. 

Magnesium-mica,  237. 
Magnetic  iron-ore,  123. 

• properties,  44,  124. 

pyrites,  94. 

Magnetite,  123. 
Magnets,  natural,  124. 
Malachite,  154. 
Malformation  of  crystals,  16. 
Mamillary  forms,  31. 
Manganese  carbonate,  145.  i 

ores,  51. 

oxides,  134.  > 

silicate,  207.  * 

tungstate,  167.  * 

Manganese-spar,  145. 


Manganite,  134.    ' 
Marcasite,  91. 
Massive  minerals,  29. 
Matter,  kinds  of,  5. 
Meerschaum,  242. 
Mercury,  native,  88. 

sulphide,  88. 

Metallic  elements,  47,  54. 

luster,  38. 

ores,  51. 

Metals,  42,  47. 
Meta-silicic  acid,  184. 
Meteoric  iron,  77. 
Meteorites,  219. 

diamond  in,  60. 

Methylene  iodide,  40. 
Mica  group,  233. 
Mica-schist,  234. 
Micaceous  cleavage,  42,  234. 

iron-ore,  122. 

Microcline,  187,  191. 
<Millerite,  26. 
Mimetite,  174. 
Mineral,  definition  of,  2. 

kingdom,  i. 

mixtures,  3. 

species,  2. 

substances,  3. 

Mineral-pitch,  255. 
Mineralogy,  i. 
Minerals,  forms  of,  6. 

,  simple,  2. 

Mispickel,  89. 
Mis-shapen  crystals,  16. 
Mixed  crystals,  50. 
Mixtures  of  minerals,  3. 
Mocha-stone,  118. 
Models  of  crystals,  10. 
Mohs's  scale  of  hardness,  43. 
Molybdates,  156. 
Molybdenite,  81. 
Molybdenum  sulphide,  8r. 
Monoclinic  felspars,  186. 
pyroxenes,  203. 


268 


INDEX 


Monoclinic  system,  8,  22,  163. 
Moon-stone,  190. 
Moss-agate,  118. 
Mossy  aggregates,  31. 
Mountain-cork,  30. 
Mountain-leather,  30. 
Miiller's  glass,  119. 
Muscovite,  236. 
Muscovy  glass,  236 

Nail-head-spar,  140. 

Native  elements,  53. 

Natrolite,  247. 

Natural  history  classification,  50. 

Needle-like  crystals,  26. 

Needle-stone,  247. 

Negro-heads,  232. 

Nephrite,  199. 

New  Zealand  greenstone,  201. 

Niccolite,  87. 

Nickel  arsenate,  177. 

arsenides,  87,  97. 

ores,  51. 

Nickel-bloom,  97,  177. 
Nickel-iron,  77. 
Niobates,  249. 
Nitrogen,  46. 
Nodular  forms,  30,  31. 
Non-metallic  elements,  47,  54, 
Norite,  193. 
Nuggets  of  gold,  72. 

Oblique  system,  22. 
Obtuse  pyramid,  19. 

rhombohedron,  24. 

Occidental  "  topaz  "  (quartz),  115. 

Ochre,  134. 

Octahedral  cleavage,  42. 

habit,  27. 

Octahedron,  9,  19. 
Oligoclase,  192. 
Olivine,  218. 

(garnet),  217. 

Onyx,  118. 


Onyx-marble,  149. 
Oolite,  139. 
Opacity,  39. 
Opal,  119. 
Opalescence,  119. 
Optical  properties,  34,  56. 
Ores,  51,  78. 
Organic  bodies,  I. 

compounds,  183. 

substances,  252. 

Oriental      "  amethyst "       (corun- 
dum), 35. 

"emerald  "  (corundum),  35. 

"  topaz  "  (corundum),  35. 

Orpiment,  81. 
Orthoclase,  187,  189. 
Orthorhombic  carbonates,  150. 

pyroxenes,  203. 

system,  8,  21,  157. 

Ortho-silicic  acid,  184. 
Osram  electric  lamps,  168. 
Osseous  amber,  254. 
Oxides,  no. 
Oxygen-salts,  50,  138. 

Palaeontology,  i. 
Paper-spar,  140. 
Paragonite,  235. 
Parallel  fibrous  structure,  30. 

grouping,  27. 

Parallelism  of  edges,  17. 
Peacock-ore,  96. 
Pearls,  2. 
Pea-stone,  149. 
Pegmatite,  189. 
Pencil-ore,  122. 
Penninite,  239. 

Pentagonal-dodecahedron,  93. 
Percussion  figure,  235. 
Peridot,  219. 
Perovskite,  250. 
Petrology,  I. 
Phillipsite,  246. 
Phlogopite,  239. 


INDEX 


269 


Phosphates,  170. 
Phosphorescence,  104. 
Phosphorite,  172. 
Physical  characters,  34. 
Pinacoid,  20,  21. 
Pingos  d'agua,  220. 
Pisolite,  149. 
Pisolitic  structure,  31. 
Pistacite,  226. 
Pitch,  255. 
Pitch-opal,  120. 
Pitchblende,  46,  132. 
Plagioclase,  187,  192. 
Planes  of  symmetry,  10. 
Plant  remains,  258. 
Plasma,  117. 
Platinum,  71. 
Platy  crystals,  26,  234, 
Pleochroism,  37,  224. 
Plumbago,  64. 
Polarized  light,  231. 
Polonium,  133. 
Polymorphism,  49. 
Potash-felspar,  186,  191. 
Potassium-mica,  235. 
Pot-stone,  243. 
Practical  uses,  51. 
Precious  opal,  120. 

stones,  52. 

Prehnite,  227. 
Princess  Blue,  208. 
Prism,  20,  21. 
Prismatic  habit,  26. 
Proustite,  100. 
Prussian  amber,  254. 
Pseudo-crocidolite,  202. 
Pseudo-hexagonal  crystal,  28. 
Pseudomorphs,  32,  154,  202. 
Psilomelane,  134. 
Pyramid,  20. 
Pyramidal  habit,  27. 
Pyrargyrite,  99. 
Pyrite,  92. 
Pyrites,  92. 


Pyrites,  arsenical,  89. 

,  cockscomb-,  91. 

— ,  copper-,  95. 

,  iron-,  92. 

,  magnetic,  94. 

,  spear-,  91. 

Pyroelectricity,  230. 
Pyrolusite,  134. 
Pyromorphite,  173. 
Pyrope,  213,  216. 
Pyroxene  group,  202. 
Pyrrhotite,  94. 

Quartz,  no. 

cat's-eye,  116. 

sand,  no,  118. 

Quicksilver,  88. 

Radially  fibrous  structure,  31. 
Radio-activity,  133. 
Radium,  133,  179. 
Rainbow  colors,  120. 
Raspberry-spar,  146. 
Realgar,  80. 
Red  jasper,  117. 
Red-copper-ore,  136. 
Red-iron-ore,  122. 
Red-silver-ores,  99. 
Re-entrant  angles,  28. 
Reference  axes,  9. 
Refraction,  double,  142. 
Refractive  index,  56. 
Regular  forms,  6. 

grouping  of  crystals,  27. 

(=  cubic)  system,  9. 

Reniform,  31. 
Resin,  fossil,  253. 
Resinous  luster,  38. 
Rhodochrosite,  145. 
Rhodolite,  216. 
Rhodonite,  207. 
Rhombic  system,  8. 
Rhombic-dodecahedron,  13. 
Rhombohedral  carbonates,  143. 


270 


INDEX 


Rhombohedral  cleavage,  41. 

system,  8,  23,  139,  229. 

Rhombohedron,  23. 

Rhomb-spar,  140. 

Riband- jasper,  116. 

Right-handed  crystals,  112. 

Rock-cavities,  fillings,  31. 

Rock-crystal,  in,  118. 

Rock-drills,  61. 

Rock-forming  minerals,  52,  186. 

Rock-phosphate,  172. 

Rock-salt,  105. 

Rocks,  4. 

Rod-like  crystals,  27. 

Rose-cut,  61. 

Rose-garnet,  217. 

Rose-quartz,  115. 

Rose-topaz,  220. 

Rounded  forms,  30,  31. 

Rubellite,  229. 

Ruby,  126. 

,  Arizona  (garnet),  216. 

,  balas-   (spinel),  125. 

,  Cape  (garnet),  216. 

,  Elie  (garnet),  216. 

,  spinel-,  125. 

Ruby-copper,  136. 
Ruby-silver-ore,  99. 
Rutile,  49. 

Salt  crystals,  5. 

,  common,  105. 

Salts,  51,  101. 
Saltpetre  crystals,  6. 
Sancy  diamond,  61. 
Sand,  no. 
Sandstone,  no,  233. 
Sanidine,  190. 
Sapphire,  126. 
Sard,  117. 
Satin-spar,  165. 
Scale  of  hardness,  43. 
Scalenohedron,  25. 
Scaly  crystals,  26,  234. 


Scheelite,  169. 
Schorl,  229. 

Secondary  minerals,  244. 
Selenite,  164. 
Semi-metals,  67. 
Sepiolite,  242. 
Serpentine,  219,  240. 
Serpentine-asbestos,  241. 
Shapes  of  crystals,  7. 
Shelly  structure,  31. 
Siderite,  144. 
Silica,  no,  183. 

,  hydrated,  1 19. 

Silica-glass,  118. 
Silicates,  183. 
Siliceous  sinter,  121. 
Silicic  acid,  184. 
Silicon  compounds,  184. 
Silky  luster,  30,  38. 
Sillimanite,  223. 
Silver  chloride,  109. 

,  horn-,  109. 

,  native,  70. 

ores,  51. 

sand,  118. 

sulph-antimonite,  99. 

sulph-arsenite,  100. 

in  lead  ore,  86. 

Simple  forms  of  crystals,  14. 

minerals,  2. 

Sinter,  siliceous,  121. 

Six-fold  symmetry,  25. 

Smalt,  98. 

Smaltite,  97. 

Smell,  45. 

Smoky-quartz,  115. 

Soap-stone,  243. 

Soapy  feel,  45. 

Soda-felspar,  186,  192. 

Sodalite  group,  207. 

Sodium  aluminium  fluoride,  109. 

silicates,  186,  207,  244,  &c. 

chloride,  105. 

Sodium-mica,  235. 


INDEX 


271 


Soils,  170,  188,  245. 
Spar,  52. 

Sparry  minerals,  52. 
Spathic  iron-ore,  144. 
Spear-pyrites,  91. 
Specific  gravity,  38. 
Spectrum,  56. 
Specular  iron-ore,  121. 
Sperrylite,  72. 
Spessartite,  213. 
Sphalerite,  84. 
Sphene,  249. 
Spherical  forms,  31. 
Spinel,  125. 
Spinel-ruby,  125. 
Splintery  fracture,  43. 
Spodumene,  206. 
Square  prism,  20. 
Stalactites,  142. 
Stalactitic  forms,  31. 
Star-like  groups,  30. 
Star-ruby,  128. 
Star-sapphire,  128. 
Statuary  marble,  3,  29. 
Staurolite,  232. 
Steatite,  243. 
Stibnite,  78. 
Stilbite,  248. 
Stoff,  5. 
Stones,  3. 
Strahlstein,  198. 
Streak,  36. 
Stream-tin,  130. 
Striated  felspars,  188. 
Striations  on  crystals,  113. 
Strontianite,  150. 
Strontium  sulphate,  160. 
Structure,  29. 

Sub-conchoidal  fracture,  43. 
Succinite,  255. 
Sugar  crystals,  6,  29. 
Sulphates,  156. 
Sulphides,  78. 
Sulphur,  48,  64. 


Sulphur-salts,  78. 
Sulphuric  acid,  67,  94,  156.  ' 
Sun-stone,  193. 
Surfaces  of  fracture,  41. 
Swallow-tail  twin,  28. 
Symbols,  chemical,  46. 

of  crystal  faces,  13. 

Symmetry  of  crystals,  10. 

,  axes  of,  ii. 

,  center  of,  12,  23. 

,  planes  of,  10. 

Systems  of  crystals,  8. 

Tabular  habit,  27. 

spar,  206. 

Talc,  243. 

-  (mica),  237. 
Tantalite,  251. 
Tantalum,  251. 
Taste,  45. 

Tellurides  of  gold,  74, 
Tetrad  axis,  n. 
Tetragonal  system,  8,  19. 
Tetrahedral  crystals,  83,  98. 
Tetrahedrite,  98. 
Tetrahedron,  83,  98. 
Texture,  29. 

Three-fold  symmetry,  25. 
Thumb-marked  fracture,  114. 
Tiger-eye,  116,  202. 
Tile-ore,  137. 
Tin-oxide,  129. 
Tin-stone,  129. 
Tinder-boxes,  93. 
Titanates,  249. 
Titanite,  250. 
Titanium  dioxide,  49. 
Titano-silicates,  249. 
Topaz,  220,  35. 

,  false  (quartz),  115. 

.occidental  (quartz),  115. 

,  oriental  (corundum),  35. 

Torbernite,  178. 
Touch,  45. 


272 


INDEX 


Tourmaline,  228. 

tongs,  231. 

Translucency,  38. 
Transparency,  37. 
Tree-like  forms,  31. 
Tremolite,  197. 
Triad  axis,  12. 
Triangular  pyramid,  229. 
Triclinic  felspars,  186. 

(=  anorthic)  system,  23. 

Trigonal  prism,  229. 
Trimorphism,  49,  223. 
Tripolite,  121. 
Tungstates,  156. 
Tungsten,  168. 

ores,  52. 

Turamali,  231. 
Turquoise,  181. 
Twin-lamellae,  187. 
Twin-plane,  28. 
Twinned  crystals,  28,  102. 
Twinning,  secondary,  225. 
Twins,  28. 

Ultramarine,  209. 

Uralian  "  emerald"  (garnet),  217. 

Uraninite,  133. 

Uranium  micas,  178. 

ores,  52. 

oxide,  133. 

phosphate,  179. 

Uvarovite,  213,  217. 


Vanadates,  170. 


Vanadinite,  175. 
Variegated  lead-ore,  174. 
Vesuvianite,  220.  . 
Vitreous  luster,  38. 
Vitriol,  oil  of,  67,  94,  156. 
Vivianite,  180. 

Water  of  crystallization,  244. 

of  diamond,  57. 

Wavellite,  177. 
Wax-opal,  120. 
Waxy  luster,  38. 
Whewellite,  252. 
White  amphibole,  197. 

arsenic,  90. 

lead-ore,  150. 

Wiry  forms,  31. 
Witherite,  150,  159. 
Wolframite,  167. 
Wollastonite,  206. 
Wood-tin,  129. 
Writing  sand,  108. 
Wulfenite,  168. 

Zeolite  group,  244. 
Zinc  carbonate,  143. 

ores,  51. 

sulphide,  83. 

Zinc-blende,  83. 
Zinc-spinel,   125. 
Zinnwaldite,  238. 
Zircon,  130. 
Zones  of  colors,  231. 
on  crystals,  17. 


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